Text, Pixels, or Both? Evaluating Input Representations for Multimodal Document QA

# Text, Pixels, or Both? Evaluating Input Representations for Multimodal Document QA
Source: [https://arxiv.org/html/2609.22628](https://arxiv.org/html/2609.22628)
Sepideh KharaghaniSaverio VadacchinoAffiliation:Alejandro PosadaKurt MacDonaldYing ZhangAffiliation:ServiceNow CanadaAffiliation:\{nikhilreddy\.pottanigari, sepideh\.kharaghani,Affiliation:saverio\.vadacchino, alejandro\.posada,Affiliation:kurt\.macdonald, yin\.zhang\}@servicenow\.com

###### Abstract

Every document QA system begins with a choice that is rarely studied on its own: whether to feed the model page images, extracted text, or both\. We isolate this choice, holding the prompt, judge, and scoring pipeline fixed, across four commercial model endpoints, two corpora, and two context regimes \(gold evidence pages and the full document\)\. On documents that fit the image budget, page images lead on accuracy at every document length on both corpora, but this advantage carries a growing latency and cost premium: text latency stays roughly flat as documents lengthen while image latency rises steadily\. Text and images also fail on different questions, with exactly one representation correct on19–25%of items across the reported cells, so neither subsumes the other\. Exploiting this complementarity, a lightweight TF\-IDF router that reads only the question text gains2\.6 pointsover always\-text while cutting median latency30%relative to always\-vision, on a document\-disjoint held\-out split\.

## 1Introduction

Before a document QA system can answer a question, a simple preprocessing choice sets how the document reaches the model: as page images, as extracted text, or both\([Zhang et al\., 2024](https://arxiv.org/html/2609.22628#bib.bib12)\)\. A knowledge\-base article may encode evidence in a screenshot, diagram, or spatial arrangement that OCR text preserves only partially\. Sending every page as an image retains this visual evidence but increases input size and inference latency on long documents\. Combining both modalities can be worse than either channel alone, depending on the model\.

This choice affects accuracy and latency together\. Always sending images for a long report, or text for a screenshot\-heavy knowledge base, can hurt answer quality or increase inference time\. However, representation is often evaluated together with changes to the model, prompt, retrieval method, or evaluator, making its contribution difficult to identify\.

![Refer to caption](https://arxiv.org/html/2609.22628v1/input_representation1.png)Figure 1:Three document representations use the same QA prompt and judge\.Imagepreserves layout and visual content\.Textis compact and uncapped by page count\.Text\+Imageconcatenates OCR text followed by page images at the cost of larger context\.We address this by comparingImage\(page images\),Text\(OCR text\), andText\+Image\(text then images\) across four commercial endpoints from two providers, one public benchmark\([Ma et al\., 2024b](https://arxiv.org/html/2609.22628#bib.bib3)\), and one enterprise knowledge base\. Each model runs under two retrieval regimes:Oracle\(gold evidence pages\) andFull\(full document\)\. Within each model, corpus, and regime, the prompt, judge, and scoring pipeline are held constant; the representation and its resulting input size are the intended experimental differences\.

Table 1:Evaluation corpora\. The≤\\leq50\-pagecolumns count the documents short enough to fit the vision page cap and their questions \(the subset used in Table[3](https://arxiv.org/html/2609.22628#S3.T3)\)\.Our contributions:

- •Acontrolled representation benchmarkacross four models, two corpora, and two retrieval regimes with a fixed prompt and judge\.
- •Asystematic analysis of how document length interacts with input representation, showing thatImageleads on accuracy within the image budget while their latency and cost grow with document length \(Section[4\.1](https://arxiv.org/html/2609.22628#S4.SS1)\)\.
- •Alightweight question\-text routerthat captures much of the accuracy gap to always\-vision while retaining most of text’s latency advantage, evaluated on a document\-disjoint held\-out split \(Section[5](https://arxiv.org/html/2609.22628#S5)\)\.

## 2Related Work

#### Document QA and representation\.

Document QA has expanded from single\-page visual understanding\([Mathew et al\., 2021](https://arxiv.org/html/2609.22628#bib.bib1)\)to multi\-page reasoning over tables, charts, and cross\-page evidence\([Van Landeghem et al\., 2023](https://arxiv.org/html/2609.22628#bib.bib2);[Ma et al\., 2024b](https://arxiv.org/html/2609.22628#bib.bib3);[Deng et al\., 2025](https://arxiv.org/html/2609.22628#bib.bib4)\)\. Layout\-aware models such as LayoutLMv3\([Huang et al\., 2022](https://arxiv.org/html/2609.22628#bib.bib10)\)and Pix2Struct\([Lee et al\., 2023](https://arxiv.org/html/2609.22628#bib.bib11)\)learn joint text–image representations\.[Berghaus et al\. \(2025\)](https://arxiv.org/html/2609.22628#bib.bib16)compare image\- and text\-based processing for invoice extraction and report model\- and dataset\-dependent results\. Our experiments compare representations under a common prompt and evaluation pipeline across four models, two corpora, and two regimes\.

#### Multimodal document retrieval and benchmarks\.

Retrieval\-augmented generation\([Lewis et al\., 2020](https://arxiv.org/html/2609.22628#bib.bib7)\)couples two decisions: which evidence to retrieve and how to represent it\. PDFTriage\([Saad\-Falcon et al\., 2024](https://arxiv.org/html/2609.22628#bib.bib14)\)routes questions to document structure; M3DocRAG\([Cho et al\., 2024](https://arxiv.org/html/2609.22628#bib.bib5)\)and MMDocRAG\([Dong et al\., 2025](https://arxiv.org/html/2609.22628#bib.bib6)\)retrieve multimodal document evidence\. OurOracle/Fullregimes compare gold\-page input with no page retrieval\. Long\-context position effects are established for text\([Liu et al\., 2024](https://arxiv.org/html/2609.22628#bib.bib8)\), and prompt\-compression methods reduce text context length\([Jiang et al\., 2023](https://arxiv.org/html/2609.22628#bib.bib21);[Li et al\., 2023](https://arxiv.org/html/2609.22628#bib.bib22)\)\. We examine how document representation interacts with full\-document context and a fixed image budget\. A parallel line of work asks whether page images can replace extracted text directly\. ColPali\([Faysse et al\., 2025](https://arxiv.org/html/2609.22628#bib.bib23)\)and Document Screenshot Embedding\([Ma et al\., 2024a](https://arxiv.org/html/2609.22628#bib.bib24)\)embed rendered pages rather than parsed text, and VisRAG\([Yu et al\., 2025](https://arxiv.org/html/2609.22628#bib.bib25)\)builds a vision\-language RAG pipeline over page images, each reporting that image representations avoid parsing loss on layout\-rich documents\. Our controlled comparison isolates this same text\-versus\-image choice at generation time rather than at retrieval time\.

Table 2:Evaluation metrics: accuracy as the primary outcome; pairwise disagreement, and latency
#### Parsing and evaluation\.

Text quality depends on the parsing pipeline\. Neural parsers\([Blecher et al\., 2024](https://arxiv.org/html/2609.22628#bib.bib20);[Nassar et al\., 2025](https://arxiv.org/html/2609.22628#bib.bib13)\)produce structured markup from page images, while direct multimodal processing can retain non\-textual evidence that text extraction omits\([Shen et al\., 2026](https://arxiv.org/html/2609.22628#bib.bib15)\)\. Our text condition uses a proprietary pipeline that outperforms Tesseract\([Smith, 2007](https://arxiv.org/html/2609.22628#bib.bib9)\)on three OCR datasets \(Appendix Table[8](https://arxiv.org/html/2609.22628#A1.T8)\); this comparison does not eliminate extraction error as a possible source of the text–image gap\. We use the same LLM judge and prompt in every condition, while recognizing that LLM\-based evaluation can itself introduce error\([Zheng et al\., 2023](https://arxiv.org/html/2609.22628#bib.bib17);[Liu et al\., 2023](https://arxiv.org/html/2609.22628#bib.bib18);[Bavaresco et al\., 2025](https://arxiv.org/html/2609.22628#bib.bib19)\)\.

## 3Setup

#### Representation strategies\.

Imagesends each page as a 300\-DPI image;Textsends page text from a proprietary OCR pipeline whose comparison with Tesseract\([Smith, 2007](https://arxiv.org/html/2609.22628#bib.bib9)\)appears in Appendix Table[8](https://arxiv.org/html/2609.22628#A1.T8);Text\+Imagesupplies the OCR text followed by the page images\. All three use the same QA prompt \(Appendix[E](https://arxiv.org/html/2609.22628#A5)\)\.

#### Datasets\.

Two corpora span short, visually dense articles and longer mixed\-format documents \(Table[1](https://arxiv.org/html/2609.22628#S1.T1)\)\. The enterprise knowledge base \(KB\) averages 5\.7 pages per document and has 93\.6% of its questions labeled as requiring visual or cross\-modal evidence\. MMLongBench\-Doc\([Ma et al\., 2024b](https://arxiv.org/html/2609.22628#bib.bib3)\)contains longer PDFs \(48\.3 pages on average\) with single\-page, cross\-page, and unanswerable questions\. The KB is internal, whereas MMLongBench\-Doc provides a public comparison corpus \(Appendix[B](https://arxiv.org/html/2609.22628#A2)\)\.

Table 3:Accuracy on the≤\\leq50\-page subset\(documents short enough that every representation sees the full page set, i\.e\. no vision\-input truncation\)\. Cells show mean±\\pmstd across runs, quantifying endpoint and judge nondeterminism rather than question\-level uncertainty; the*Avg\.*row is the arithmetic mean of the four model means per column\. Left block:Oracle\(gold evidence pages\); right block:Full\(full document\)\.
#### Models and retrieval regimes\.

We evaluate four models from two providers: Gemini 3\.1 Flash\-Lite and Gemini 3\.1 Pro \(Google via Vertex AI\), GPT\-5\.4 and GPT\-5\.4\-mini \(OpenAI via Azure\)111Exact API identifiers in Appendix[A](https://arxiv.org/html/2609.22628#A1)\.\. Each model runs under two regimes that bracket realistic retrieval:

- •Oraclesupplies only the gold evidence pages\. It measures answer generation when annotated evidence pages are provided, without evaluating page retrieval\.
- •Fullsends the full document with no retrieval\. Here the model has to both find the relevant pages and extract the answer from a much larger, noisier context\.

The gap between regimes measures sensitivity to replacing annotated evidence pages with the full document: a large positive gap indicates that the representation benefits from page selection, whereas a small or negative gap indicates comparable or better performance with full context\.

Table 4:Mean latency \(seconds/question,±\\pmstd across runs\) on the≤\\leq50\-page subset, where no representation is truncated by the vision page cap\. Left block:Oracle\(gold evidence pages\); right block:Full\(full document\)\. The*Avg\.*row is the arithmetic mean of the four model means per column\. Lower is better\.
#### Page cap and document scope\.

Due to model\-provider limits, we cap all requests at50 page imagesand restrict the main analysis to documents of 50 pages or fewer, so that every representation sees the complete page set\. We adopt this restriction specifically to isolate the page\-cap truncation effect from any genuine modality effect\. Applying the same cap to both providers keeps theImageconditions comparable\. For completeness, Appendix[C](https://arxiv.org/html/2609.22628#A3)reports results over*all*documents, still capped at 50 page images\. In that setting, documents longer than 50 pages are truncated underImage, whileTextcontinues to see the full document, and the two effects are shown to be separable\.

#### Scoring and metrics\.

Models produce free\-form answers with a 3,000\-token limit, temperature 0, and a fixed seed of 30\. For each model, representation, dataset, and regime, we average correctness overfive prediction runs\. Each prediction is then evaluated by a fixed GPT\-5\.2 judge overthree judging runs, and we report the mean score\. Per\-cell run\-to\-run standard deviations are reported alongside every accuracy and latency value in Tables[3](https://arxiv.org/html/2609.22628#S3.T3)and[4](https://arxiv.org/html/2609.22628#S3.T4)\. Reported±\\pmvalues are standard deviations across the five prediction and three judging runs\. Because decoding is greedy with a fixed seed, they quantify residual nondeterminism in the hosted endpoints and the judge rather than sampling uncertainty over questions\. Table[2](https://arxiv.org/html/2609.22628#S2.T2)defines the three metrics\.

![Refer to caption](https://arxiv.org/html/2609.22628v1/fig_bucket_main.png)Figure 2:Accuracy \(top\) and median latency \(bottom\) versus document\-length bucket on the≤\\leq50\-page subset underFullretrieval, aggregated across all four models\. The Enterprise KB has no 40–50 page bucket, as its documents are shorter \(median 4 pages\)\. Bands show±\\pmstd across models\.

## 4Results

We organize results around two questions: how the accuracy–latency trade\-off changes with document length, and whether text and images miss the same questions\. Table[3](https://arxiv.org/html/2609.22628#S3.T3)and Figure[2](https://arxiv.org/html/2609.22628#S3.F2)show that on the≤\\leq50\-page subset, image\-bearing inputs \(Image,Text\+Image\) lead on accuracy across document lengths on both corpora, whileTextstays consistently lower but at a fraction of the latency: text latency is roughly flat in document length, whereasImageandText\+Imagelatency grows steadily\. The accuracy gap narrows on the shortest and longest KB buckets but does not close\. Section[5](https://arxiv.org/html/2609.22628#S5)then exploits the resulting complementarity with a lightweight router\.

### 4\.1Accuracy–latency trade\-off across representations

Page images retain layout and non\-textual content that extracted text may omit\([Shen et al\., 2026](https://arxiv.org/html/2609.22628#bib.bib15)\), but any accuracy gain must be weighed against latency\. We examine this trade\-off under two regimes, on the≤\\leq50\-page subset where every representation sees the complete page set\.

#### Oracle: image inputs lead with gold pages\.

UnderOracle, eitherImageorText\+Imageis best for every model on both corpora, with gains overTextof up to 14\.8 percentage points on the KB and 11\.6 points on MMLongBench\-Doc \(Table[3](https://arxiv.org/html/2609.22628#S3.T3)\)\. This is consistent with image inputs retaining evidence omitted by the text pipeline, but it does not identify whether the difference arises from visual evidence, OCR errors, or model\-specific processing\. The gain comes at a latency cost: on the KB, image input raises average latency from 3\.0 s \(Text\) to 3\.9 s \(Image\) and 3\.7 s \(Text\+Image\), and on MMLongBench\-Doc from 3\.5 s to 5\.9 s \(Table[4](https://arxiv.org/html/2609.22628#S3.T4)\)\. The largerOraclelatency gap on MMLongBench\-Doc is consistent with its questions citing more evidence pages per query \(1\.88 vs\. 1\.05 on the KB \(Appendix[B](https://arxiv.org/html/2609.22628#A2)\)\), so eachImageorText\+Imagerequest carries more page images\.

#### Full: image accuracy leads, but the latency gap widens\.

UnderFull, image\-bearing inputs still lead on accuracy on both corpora: on the KB,Text\+ImageandImageaverage 0\.72 and 0\.70 versus 0\.63 forText; on MMLongBench\-Doc,Imageaverages 0\.66 versus 0\.57 forText\(Table[3](https://arxiv.org/html/2609.22628#S3.T3)\)\. Figure[2](https://arxiv.org/html/2609.22628#S3.F2)showsTextlatency staying roughly flat in document length \(∼\{\\sim\}3 s on the KB,∼\{\\sim\}3–4 s on MMLongBench\-Doc\), whileImageandText\+Imagelatency rise steadily, reaching∼\{\\sim\}9 s on the longest KB bucket and∼\{\\sim\}12 s on the longest MMLongBench\-Doc bucket\. Accuracy, by contrast, remains image\-favored in every length bucket within the subset\. The bucket\-level view is included to show that the accuracy ordering does not reverse within the un\-truncated range; the reversal reported in Appendix[C](https://arxiv.org/html/2609.22628#A3)appears only once the page cap binds\.

Table 5:Average cost \(USD per 100 questions\) and latency \(seconds per question\) per representation strategy, averaged across the four models and two datasets\. Full per\-model, per\-dataset latency breakdowns are in Table[4](https://arxiv.org/html/2609.22628#S3.T4)\.
#### Does combining both modalities help?

WhetherText\+Imagehelps depends on the endpoint and corpus\. GPT\-5\.4 and GPT\-5\.4\-mini gain roughly 5 pp \(5\.6 and 4\.8, respectively\) fromText\+ImageoverImageon the KB underOracle, and theirImage\-vs\-Text\+Imagedisagreement is the highest among the KB cells \(13–14%, Table[6](https://arxiv.org/html/2609.22628#S4.T6)\), showing that the additional text changes some correctness outcomes\. Gemini Pro’sImageandText\+Imageoutcomes differ less often \(∼\{\\sim\}7–8% on the KB\)\. On MMLongBench\-Doc/Full, Gemini Pro’sText\+Imagefalls behind itsImageresult and roughly ties itsTextresult \(0\.623 vs\. 0\.690 and 0\.628\), while for the GPT endpointsText\+Imageis the most accurate condition on the KB\.Text\+Imageprovides no consistent latency benefit and is the slowest condition on the longer corpus \(Table[4](https://arxiv.org/html/2609.22628#S3.T4)\)\.

Table 6:Pairwise disagreement rate \(%\) on the≤\\leq50\-page subset: share of questions where exactly one format is correct \(i\.e\., the sum of text\-only\-correct and vision\-only\-correct questions\)\. A rate of 10% in theText/Imagecolumn means that on 10% of questions one format answered correctly and the other did not\.Coral≥20%\{\\geq\}20\\%;mint≤10%\{\\leq\}10\\%\.Finding 1\.Document length governs the*cost*of the trade\-off, not its direction\. Within the≤\\leq50\-page subset, image\-bearing inputs \(ImageorText\+Image\) lead on accuracy on both corpora, with no length bucket showing a text advantage; what grows with length is their latency, which rises steadily while text latency stays roughly flat\.

#### The image advantage carries a cost and latency premium\.

Table[5](https://arxiv.org/html/2609.22628#S4.T5)summarizes cost and latency across the four models and two corpora\. Within each regime,Textis consistently the cheapest and fastest condition: underOracle, moving fromTexttoImageraises cost 1\.5×\\times\(0\.61 to 0\.91 $/100q\) and latency from 3\.3 to 4\.9 s/q, andText\+Imageis more expensive still \(1\.29 $/100q\)\.

The premium widens underFull, where the full document is processed:ImageandText\+Imagecost 1\.9×\\timesand 2\.4×\\timesas much asText, respectively, while their latency is roughly twice that of text \(7\.0 and 7\.3 s/q vs\. 3\.7 s/q\)\. SinceImageorText\+Imagealso lead on accuracy \(Section[4\.1](https://arxiv.org/html/2609.22628#S4.SS1)\), no representation dominates on both axes\. This trade\-off motivates per\-question routing, using image input only when it is likely to improve the answer\.

### 4\.2Text and images answer different questions

TextandImagegive different correctness outcomes on approximately19–25%of questions across the reported corpus, model, and regime cells \(Table[6](https://arxiv.org/html/2609.22628#S4.T6)\)\. This disagreement decomposes into a text\-only\-correct slice and a larger vision\-only\-correct slice, reflecting vision’s higher overall accuracy\. On the short KB articles,OracleandFulldisagreement rates are nearly identical because the full document is already close to the evidence pages\. On MMLongBench\-Doc the two regimes diverge somewhat more, consistent with extra context amplifying representation differences\. Because both exclusive\-correct slices are present in every cell, an oracle that picks the correct format per question would beat either fixed policy\.

Finding 2\.Text and images answer different questions: 19–25%Text/Imagedisagreement across the reported cells\. Neither format subsumes the other; this establishes headroom for per\-question selection\.

## 5Per\-Question Representation Routing

Text and images each answer questions the other misses \(Finding 2\)\. This complementarity raises a concrete question: if a system could decide*per question*whether to send text or images, it could capture the accuracy of images where they help while keeping text’s lower latency and cost elsewhere\. We test whether this is achievable using only the question text, as a proof of concept that quantifies the potential benefit rather than a production\-ready system\.

#### Router design\.

We frame routing as binary classification\. For each \(question, model\) pair underFull, the label isy=1y=1ifImageis correct andTextis incorrect, andy=0y=0otherwise\. When both representations are correct we default toTextbecause it is faster in aggregate; when both are incorrect we also assignText, treating the example as a non\-vision\-win case\. We excludeText\+Imagefrom the router: although often competitive, it adds a second input channel and closely tracksImagein many settings \(Table[3](https://arxiv.org/html/2609.22628#S3.T3)\), so routing betweenTextandImagetargets the primary accuracy–latency trade\-off\.

Labels are obtained by running both representations on every question\. This makes the router a proof of concept: it measures how much a per\-question policy*could*gain if it selected correctly, not the performance of a deployable classifier\. In production, a similar router would be trained on a smaller labeled sample and applied to new questions\. In our training set only 13\.2% of examples are positive, soImageuniquely beatsTexton a small fraction of questions\.

We represent each question with TF\-IDF features and train a logistic regression classifier with balanced class weights, using an approximately 80/20 document\-level split stratified by dataset and model\. All questions from the same document stay in one partition, ensuring zero document leakage and a more reliable estimate of generalization to unseen documents\.

All router numbers in Table[7](https://arxiv.org/html/2609.22628#S5.T7)are computed on this 20% held\-out test partition over the*full corpus*\(all documents, image budget still capped at 50 pages\) and pooled across the four models, so they are micro\-averages over pooled questions rather than the per\-model,≤\\leq50\-page macro\-averages in Table[3](https://arxiv.org/html/2609.22628#S3.T3); the two are not directly comparable\. Always\-vision is correspondingly lower on MMLongBench\-Doc here, because the held\-out set includes documents beyond the 50\-page cap where image inputs are truncated \(Appendix[C](https://arxiv.org/html/2609.22628#A3)\)\.

Table 7:Held\-out question\-text router results underFull, pooling all four models\. The test partition is the 20% split described in the text\.*Routing accuracy*= accuracy when every question uses the modality the strategy selects \(cf\. per\-model accuracy in Table[3](https://arxiv.org/html/2609.22628#S3.T3)\)\.*Implied med\. latency*= median wall\-clock seconds across the strategy’s per\-question routing decisions \(cf\. per\-model latency in Table[4](https://arxiv.org/html/2609.22628#S3.T4)\)\. Gains are computed from unrounded scores\.Bold= best per column within each dataset\.
#### Accuracy\.

On the document\-disjoint split, always\-vision remains the strongest fixed policy, reaching 69\.2% on KB and 58\.0% on MMLongBench\-Doc \(Table[7](https://arxiv.org/html/2609.22628#S5.T7)\)\. The router does not beat it, but closes much of the gap over always\-text, increasing accuracy by2\.7 ppon KB \(65\.0→\\to67\.8%\) and2\.0 ppon MMLongBench\-Doc \(53\.9→\\to55\.9%\) while routing 26\.8% of examples to vision\. Overall, the router reaches 66\.2%, gaining2\.6 ppover always\-text while remaining 1\.5 pp below always\-vision\.

#### Latency\.

Since the router keeps most questions on the faster text path, median latency drops substantially relative to always\-vision: from 4\.4 s to 3\.1 s overall, a savings of approximately30%, while routing only 26\.8% of examples to vision\.

Finding 3\.On the held\-out split with no document overlap, a lightweight TF\-IDF router gains 2\.0–2\.7 pp over always\-text while cutting median latency by 30% relative to always\-vision\. Because training labels require running both representations on every question, this is a proof\-of\-concept estimate of achievable headroom, not a deployable classifier\.

## 6Discussion

#### Document length as the organizing variable\.

Document length organizes the accuracy–latency trade\-off, but not by changing which representation is most accurate\. On the≤\\leq50\-page subset,ImageorText\+Imageleads on accuracy across all document lengths on both corpora; what scales with length is their cost and latency, which grow steadily while text stays roughly flat\. The apparent accuracy crossover in favor of text on long documents is a consequence of the fixed 50\-image cap truncating image inputs, and appears only in the full\-corpus results \(Appendix[C](https://arxiv.org/html/2609.22628#A3)\); it is a property of the current deployment budget, not of the modality\. Complementary errors provide a separate motivation for routing: text and images disagree on roughly a fifth to a quarter of questions in every cell, so neither representation subsumes the other\. The current router, however, is evaluated only underFull, and we treat it as a proof of concept rather than a tuned system\.

#### Multimodal fusion is model\-dependent, not additive\.

Sending both text and images does not uniformly help\. The GPT endpoints gain approximately 5 pp over image\-only input on the KB underOracle, whereas the Gemini endpoints show smaller or less consistent changes \(Section[4\.1](https://arxiv.org/html/2609.22628#S4.SS1.SSS0.Px3)\)\. These outcome differences do not reveal the models’ internal fusion mechanisms, but they show that the preferred representation depends on the endpoint as well as the document setting\.

#### Implications for RAG pipelines\.

The results motivate evaluating both text and image indices rather than assuming that one representation is uniformly best\. The TF\-IDF router is a proof of concept; a stronger evaluation would use realistic retrieval, and document features in addition to the question text\.

## 7Conclusion

We compared text, image, and combined document inputs under a common prompting and evaluation pipeline across four model endpoints, two corpora, and two context regimes\. Image\-bearing inputs outperform text on both corpora, with the largest gains on the short, visually dense Enterprise KB, while text remains cheaper and faster per question\. The two representations also produce complementary correctness outcomes, with approximately 19–25% pairwise disagreement across the reported cells\. On a held\-out document\-level split with no train–test leakage, a lightweight TF\-IDF question router recovers 2\.0–2\.7 percentage points over always\-text while reducing median latency by 30% relative to always\-vision\.

## Limitations

First,OracleandFulldo not evaluate a realistic top\-kkretriever\. They compare annotated evidence pages against full\-document input, so the results bound two extremes and do not establish performance for intermediate retrieval quality\.

Second, we report run\-to\-run variation but not question\-level uncertainty\. Our design fixes the question set and varies only prediction and judging runs, so the reported standard deviations capture endpoint and judge nondeterminism\. Paired significance testing over the per\-question correctness scores would sharpen these comparisons\.

Third, theTextcondition depends on our OCR pipeline\. The pipeline outperforms Tesseract on standard document\-OCR benchmarks \(Appendix Table[8](https://arxiv.org/html/2609.22628#A1.T8)\) but is not state of the art, and may extract text less faithfully than the frontier models themselves would from the same pages\. Part of the text–image gap we report may therefore reflect extraction error rather than an intrinsic limitation of the text modality\.

## Ethics Statement

The Enterprise KB contains internal product\-support documentation owned by the authors’ organization and was accessed under existing internal data\-use policies\. Questions were created and reviewed by internal annotators as part of their regular work\. All model endpoints, including the judge, were accessed under enterprise agreements with Vertex AI and Azure OpenAI, which exclude submitted prompts and outputs from provider model training\. No data was retained beyond the terms of these agreements\. The Enterprise KB and OCR pipeline cannot be released because they are proprietary\. We therefore include MMLongBench\-Doc to enable independent reproduction of the main text\-versus\-image comparison\.

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## Appendix AExperimental Settings and Provenance

#### Input and inference\.

Images use300\-DPI PDF page renders\. Text usespage text, a proprietary OCR pipeline outperforming Tesseract\([Smith, 2007](https://arxiv.org/html/2609.22628#bib.bib9)\)\(Table[8](https://arxiv.org/html/2609.22628#A1.T8)\)\. All runs use the same QA prompt \(Appendix[E](https://arxiv.org/html/2609.22628#A5)\), no learned retriever, seed 30, a3,000\-token answer limit, andat most 50 page imagesper request\. Temperature is 0 for all four models\.

Table 8:Direct OCR comparison against Tesseract\. Scores report End2End recall under case\-sensitive matching\.Table 9:Effect of restricting to the≤\\leq50\-page subset\. Accuracy is mean±\\pmstd across runs;Δ=acc≤50−accall\\Delta=\\mathrm\{acc\}\_\{\\leq 50\}\-\\mathrm\{acc\}\_\{\\mathrm\{all\}\}\.BoldΔ\\Deltamarks\|Δ\|≥0\.02\|\\Delta\|\\geq 0\.02\. Large positiveΔ\\Deltaconcentrates inImage/Full, reflecting vision\-input truncation on documents longer than the page cap\.Table 10:Effect of the≤\\leq50\-page restriction on latency \(seconds/question, mean±\\pmstd\);Δ=lat≤50−latall\\Delta=\\mathrm\{lat\}\_\{\\leq 50\}\-\\mathrm\{lat\}\_\{\\mathrm\{all\}\}\(negative = subset is faster\)\.BoldΔ\\Deltamarks\|Δ\|≥0\.5\|\\Delta\|\\geq 0\.5s\. Large negativeΔ\\Deltaconcentrates inImage/Text\+ImageunderFull: the excluded long documents are the ones that inflate vision latency\.
#### Latency measurement\.

Provider\-side automatic retries are disabled for all API calls\. Reported latency is the wall\-clock time of the single call that produced the answer, excluding any internal retry overhead the provider would otherwise add\.

#### Judging\.

GPT\-5\.2judges answers acrossthree runs\(8,000\-token limit, two parallel calls per batch\)\([Zheng et al\., 2023](https://arxiv.org/html/2609.22628#bib.bib17);[Liu et al\., 2023](https://arxiv.org/html/2609.22628#bib.bib18);[Bavaresco et al\., 2025](https://arxiv.org/html/2609.22628#bib.bib19)\)\. Questions are batched in fixed document order\.

#### Model Identifiers\.

Run identifiers aregemini\-3\.1\-flash\-lite,gemini\-3\.1\-pro\-preview,gpt\-5\.4, andgpt\-5\.4\-mini\. These are the exact strings passed at inference time\.

## Appendix BDataset Details

#### MMLongBench\-Doc\.

MMLongBench\-Doc contains 135 PDFs with 6,522 pages, averaging 48\.3 pages per document \(median 28, range 9–468\)\([Ma et al\., 2024b](https://arxiv.org/html/2609.22628#bib.bib3)\)\. We use the current 1,091\-question release\. Questions include 485 single\-page \(44\.5%\), 360 cross\-page \(33\.0%\), and 246 unanswerable \(22\.5%\)\. On answerable questions, the evidence labels are multi\-label: Pure\-text 298 \(35\.3%\), Table 218 \(25\.8%\), Chart 178 \(21\.1%\), Image 293 \(34\.7%\), and Layout 119 \(14\.1%\)\. Answer formats include Integer 34\.2%, String 29\.5%, Float 18\.9%, and List 17\.3%\. Questions cite 1\.88 evidence pages on average \(9\.08 distinct evidence pages per document\)\.

#### Enterprise KB \(internal\)\.

The enterprise KB contains 1,447 documents \(8,296 pages; avg\. 5\.7 pages/doc\) with 5,169 human\-reviewed questions \(avg\. 3\.6 questions/doc\)\. Answer mode: visual 3,314 \(64\.1%\), cross\-modal 1,527 \(29\.5%\), textual 328 \(6\.3%\)\. Difficulty: easy 3,731 \(72\.2%\), medium 1,425 \(27\.6%\), hard 13 \(0\.3%\)\. 93\.6% of questions require visual or cross\-modal interpretation\. Questions cite 1\.05 evidence pages on average \(2\.42 distinct evidence pages per document\)\.

## Appendix CFull\-corpus results

![Refer to caption](https://arxiv.org/html/2609.22628v1/fig_bucket_appendix.png)Figure 3:Full\-corpus accuracy \(top\) and median latency \(bottom\) versus document\-length bucket underFullretrieval, aggregated across all four models\. The shaded region marks buckets above the 50\-page image cap, whereImageandText\+Imageinputs are truncated whileTextsees the full document\. Within the un\-truncated range, image\-bearing inputs lead on accuracy at every length; the crossover in favor ofTextappears only in the truncated buckets, and image latency falls there as truncation caps the number of page images processed\. Bands show±\\pmstd across models\.This appendix reports the per\-model results underlying the main text over*all*documents, with the image budget still capped at 50 pages\. Documents longer than 50 pages are therefore truncated underImageandText\+Image, whileTextsees the full document\. We report the effect of the≤\\leq50\-page restriction on accuracy \(Table[9](https://arxiv.org/html/2609.22628#A1.T9)\) and latency \(Table[10](https://arxiv.org/html/2609.22628#A1.T10)\), the full length\-bucket breakdown extending past the cap \(Figure[3](https://arxiv.org/html/2609.22628#A3.F3)\)\.

#### The restriction isolates the page\-cap truncation effect\.

On the Enterprise KB, restricting to≤\\leq50 pages changes every accuracy and latency cell by less than0\.0010\.001\(Tables[9](https://arxiv.org/html/2609.22628#A1.T9),[10](https://arxiv.org/html/2609.22628#A1.T10)\): only 6 of 5,169 questions come from longer documents, so the subset and the full corpus are effectively the same measurement\. On MMLongBench\-Doc, where 25% of documents exceed the cap, the restriction has a clear and localized effect\. The accuracy gain from restricting concentrates entirely inImage/Full, where it reaches\+0\.041\+0\.041to\+0\.058\+0\.058across the four models, and is an order of magnitude smaller forText/Fulland for everyOraclecell\. The latency change mirrors this: the large reductions \(−1\.0\-1\.0to−1\.6\-1\.6s\) fall onImageandText\+ImageunderFull, whileTextand allOraclecells barely move\. Both patterns identify the same cause: only the image conditions on long documents are affected by the restriction, because only they are truncated by the page cap\.

Table 11:Non\-exclusive error\-category shares \(% of incorrect answers\) by model, representation, and regime on Enterprise KB\. Left block:Oracle\(gold evidence pages\); right block:Full\(full document\)\. Columns need not sum to 100%\.Table 12:Non\-exclusive error\-category shares \(% of incorrect answers\) by model, representation, and regime on MMLongBench\-Doc\. Left block:Oracle\(gold evidence pages\); right block:Full\(full document\)\. Columns need not sum to 100%\.
#### The text advantage appears only past the cap\.

Figure[3](https://arxiv.org/html/2609.22628#A3.F3)extends the length\-bucket view to the full corpus \(shaded region marks buckets above the 50\-page cap\)\. Within the un\-truncated range, image\-bearing inputs lead at every length, matching the main text\. The crossover in favor ofTextemerges only in the truncated buckets: on MMLongBench\-Doc,Imageaccuracy falls to∼\{\\sim\}42% at 100–150 pages and∼\{\\sim\}39% beyond 150, whileTextholds near 58%\. This is the behavior that motivates reporting the≤\\leq50\-page subset in the main text: the crossover is a property of the fixed image budget, not of the modalities, and it disappears once every representation sees the complete page set\.

## Appendix DError Category Analysis

Tables[11](https://arxiv.org/html/2609.22628#A3.T11)and[12](https://arxiv.org/html/2609.22628#A3.T12)break down errors by model, representation, and regime \(tagged by judge; Appendix[E](https://arxiv.org/html/2609.22628#A5)\)\. Categories are non\-exclusive\.

On the KB,partial\_correctnessis the most frequent reported category \(44–57%\),extra\_incorrect\_informationfollows \(18–25%\), and switching fromOracletoFullbarely changes the profile\. On MMLongBench\-Doc,answered\_when\_unanswerableis high \(36–53%\), consistent with the fact that 22\.5% of questions are unanswerable by design; this category means the model*gave*an answer to an unanswerable question \(over\-answering\), and its rate is largely stable across regimes rather than growing with theOracle→\\toFullshift\.numerical\_imprecisionis also frequent \(25–41%\); Integer and Float are common answer formats in this benchmark\.

## Appendix EPrompt Templates

### E\.1QA Prompt

All conditions use the prompt below;ImageandText\+Imageadditionally append the rendered page images after the text context block\.

QA Prompt Template[⬇](data:text/plain;base64,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)Youareanexpertquestionansweringassistant\.Yourtaskistoprovideaconciseandaccurateanswertotheuser’squestion,strictlybasedontheprovidedcontext\.Instructions:1\.Readthequestionandtheprovidedcontextcarefully\.2\.Inthe"explanation"field,thinkstep\-by\-steptodeterminehowthecontextanswersthequestion\.3\.Formulateananswerthatdirectlyaddressesthequestion\.4\.Youranswermustbederivedsolelyfromtheinformationpresentinthecontext\.Donotuseoutsideknowledgeorinventfacts\.5\.Ifthecontextdoesnotcontainenoughinformation,state"NotAnswerable"\.6\.Keeptheanswerasshortaspossible:asinglevalue,phrase,orlistofitems\.OutputFormat:\{"explanation":"<reasoning\>","answer":"<conciseanswer\>"\}Question:\{\{question\}\}Context:

### E\.2LLM\-as\-Judge Prompt

The judge receives batches of question\-answer pairs through the\{\{qa\_pairs\}\}placeholder, following reference\-guided evaluation protocols for open\-ended model outputs\([Zheng et al\., 2023](https://arxiv.org/html/2609.22628#bib.bib17);[Liu et al\., 2023](https://arxiv.org/html/2609.22628#bib.bib18);[Bavaresco et al\., 2025](https://arxiv.org/html/2609.22628#bib.bib19)\)\.

LLM\-as\-Judge Prompt Template[⬇](data:text/plain;base64,WW91IGFyZSBhbiBleHBlcnQgZXZhbHVhdG9yIGFzc2Vzc2luZyBhbnN3ZXJzCmluIGEgUXVlc3Rpb24gYW5kIEFuc3dlciBzeXN0ZW0uIEZvciBlYWNoCnF1ZXN0aW9uLWFuc3dlciBwYWlyIHlvdSByZWNlaXZlOgoxLiBBIHVuaXF1ZSBxdWVzdGlvbiBJRAoyLiBUaGUgcXVlc3Rpb24gdGV4dAozLiBUaGUgcHJlZGljdGVkIGFuc3dlcgo0LiBUaGUgZ3JvdW5kIHRydXRoIGFuc3dlcgpEZXRlcm1pbmU6CjEuIFdoZXRoZXIgdGhlIHByZWRpY3RlZCBhbnN3ZXIgaXMKICAgc2VtYW50aWNhbGx5IGVxdWl2YWxlbnQgdG8gdGhlIGdyb3VuZCB0cnV0aC4KMi4gQWxsIGFwcGxpY2FibGUgZXJyb3Igb3Igd2FybmluZyBjYXRlZ29yaWVzLgpFdmFsdWF0aW9uIEluc3RydWN0aW9uczoKLSBFdmFsdWF0ZSBiYXNlZCBvbiBzZW1hbnRpYyBtZWFuaW5nLCBub3QKICBleGFjdCB3b3JkaW5nLgotIEJlIGZsZXhpYmxlIHdpdGggcGhyYXNpbmcsIHRlcm1pbm9sb2d5LAogIHVuaXRzLCBhYmJyZXZpYXRpb25zLCBhbmQgZm9ybWF0dGluZy4KLSBNYXJrIGluY29ycmVjdCBvbmx5IGlmOiAoMSkgY29yZSBtZWFuaW5nCiAgZGlmZmVycywgKDIpIGNvbnRhaW5zIGZhY3R1YWxseSBpbmNvcnJlY3QKICBpbmZvcm1hdGlvbiwgKDMpIGNvbnRyYWRpY3RzIHRoZSBncm91bmQKICB0cnV0aCwgb3IgKDQpIG9taXRzIGtleSBpbmZvcm1hdGlvbi4KLSBGb3IgbnVtZXJpYyB2YWx1ZXM6IGFjY2VwdCBtb3JlLXByZWNpc2UKICBleHBhbnNpb25zIG9mIHJvdW5kZWQgZ3JvdW5kIHRydXRoczsgcGVuYWxpemUKICByb3VuZGluZyB0aGF0IGxvc2VzIGFjY3VyYWN5LgotIEZvciBsaXN0czogZXZlcnkgaXRlbSBtdXN0IG1hdGNoOyBvcmRlciBkb2VzCiAgbm90IG1hdHRlci4KLSBGb3IgbXVsdGlwbGUtY2hvaWNlOiBhY2NlcHQgdGhlIG9wdGlvbgogIGxldHRlciwgdGV4dCwgb3IgYm90aC4KRVJST1IgQ0FURUdPUklFUyAoaW5jb3JyZWN0IGFuc3dlcnMgb25seSk6CiAgcGFydGlhbF9jb3JyZWN0bmVzcywKICBleHRyYV9pbmNvcnJlY3RfaW5mb3JtYXRpb24sIGNvbnRyYWRpY3Rpb24sCiAgb3Zlcmx5X3ZhZ3VlLCBpbmNvcnJlY3RfeWVzX25vLAogIGFuc3dlcmVkX3doZW5fdW5hbnN3ZXJhYmxlLAogIG51bWVyaWNhbF9pbXByZWNpc2lvbiwgbWlub3Jfc3BlbGxpbmdfZXJyb3IKV0FSTklORyBDQVRFR09SSUVTIChjb3JyZWN0IGFuc3dlcnMgb25seSk6CiAgZ3JvdW5kX3RydXRoX3JvdW5kaW5nLAogIGdyb3VuZF90cnV0aF9zcGVsbGluZ19lcnJvciwKICByZWR1bmRhbnRfaW5mb3JtYXRpb24sIGV4dHJhX2luZm9ybWF0aW9uLAogIG1pc3NpbmdfaW5mb3JtYXRpb24sIG90aGVyCk91dHB1dCBGb3JtYXQ6CnsgImV2YWx1YXRpb25zIjogWwogICAgeyAicXVlc3Rpb25faWQiOiAiPGlkPiIsCiAgICAgICJpc19jb3JyZWN0IjogdHJ1ZS9mYWxzZSwKICAgICAgImV4cGxhbmF0aW9uIjogIjxwcmVkPiB2cyA8dHJ1dGg+IiwKICAgICAgImVycm9ycyI6IFt7ImNsYXNzIjogIi4uLiIsCiAgICAgICAgICAgICAgICAgICAiZXhwbGFuYXRpb24iOiAiLi4uIn1dLAogICAgICAid2FybmluZ3MiOiBbeyJjbGFzcyI6ICIuLi4iLAogICAgICAgICAgICAgICAgICAgICAiZXhwbGFuYXRpb24iOiAiLi4uIn1dCiAgICB9LCAuLi4KXSB9)YouareanexpertevaluatorassessinganswersinaQuestionandAnswersystem\.Foreachquestion\-answerpairyoureceive:1\.AuniquequestionID2\.Thequestiontext3\.Thepredictedanswer4\.ThegroundtruthanswerDetermine:1\.Whetherthepredictedanswerissemanticallyequivalenttothegroundtruth\.2\.Allapplicableerrororwarningcategories\.EvaluationInstructions:\-Evaluatebasedonsemanticmeaning,notexactwording\.\-Beflexiblewithphrasing,terminology,units,abbreviations,andformatting\.\-Markincorrectonlyif:\(1\)coremeaningdiffers,\(2\)containsfactuallyincorrectinformation,\(3\)contradictsthegroundtruth,or\(4\)omitskeyinformation\.\-Fornumericvalues:acceptmore\-preciseexpansionsofroundedgroundtruths;penalizeroundingthatlosesaccuracy\.\-Forlists:everyitemmustmatch;orderdoesnotmatter\.\-Formultiple\-choice:accepttheoptionletter,text,orboth\.ERRORCATEGORIES\(incorrectanswersonly\):partial\_correctness,extra\_incorrect\_information,contradiction,overly\_vague,incorrect\_yes\_no,answered\_when\_unanswerable,numerical\_imprecision,minor\_spelling\_errorWARNINGCATEGORIES\(correctanswersonly\):ground\_truth\_rounding,ground\_truth\_spelling\_error,redundant\_information,extra\_information,missing\_information,otherOutputFormat:\{"evaluations":\[\{"question\_id":"<id\>","is\_correct":true/false,"explanation":"<pred\>vs<truth\>","errors":\[\{"class":"\.\.\.","explanation":"\.\.\."\}\],"warnings":\[\{"class":"\.\.\.","explanation":"\.\.\."\}\]\},\.\.\.\]\}