mokshablr/gander

Source: https://github.com/mokshablr/gander

Gander 🪿

Take a gander at any file. A tiny, open source, fully offline file viewer for Android that opens PDF, Word, Excel, PowerPoint, photos, videos, audio, Markdown, text and code in one app, with zero permissions, no ads, no tracking and no internet access at all.

Every phone ships with a dozen half-viewers that bounce your documents to cloud services. Gander is the opposite: one small APK (about 15 MB) that renders everything on the device. It cannot phone home because it does not even hold the INTERNET permission.

Screenshots

Home: recents and folders	Folder browsing	PDF

Word (.docx)	PowerPoint (.pptx)	Excel (.xlsx)

Features

• One viewer for everything: documents, spreadsheets, slides, images, video, audio, Markdown, code
• Pinch zoom and smooth scrolling everywhere, with deep zoom into huge photos (tiled decoding)
• Recent files with thumbnail previews (image, video frame, PDF first page)
• Folder browsing through one-time system grants, still without any storage permission
• Share sheet and “Open with” integration: share a file from any app (chat, mail, browser) into Gander, or tap it in a file manager
• Find in document: search inside Word, Excel, slides, Markdown, text and code with match navigation
• Share and locate: send the open file to any app, or jump to its folder in the file manager
• Private by construction: no permissions, no INTERNET, no analytics, no accounts, nothing leaves the phone
• Modern Android: Material 3, dark mode, edge to edge, works on Android 8.0+

Supported formats

Legacy binary .doc and .ppt are not supported (no faithful offline renderer exists); the app explains this and suggests re-saving as .docx / .pptx. Binary .xls works.

Install

1. Download the latest APK from Releases: Gander-x.y-arm64.apk fits practically every phone from 2017 onward (use the universal APK for very old or x86 devices).
2. Copy it to your phone, tap it, and allow “install unknown apps” when asked.
3. Optional: Play Protect may warn about an unknown developer; that is what sideloaded open source looks like. Tap “Install anyway”.

Updating: install the new APK over the old one; recents and folder grants survive.

Automatic updates without a store: install Obtainium and add https://github.com/mokshablr/gander as an app source. It follows the tagged GitHub releases here and updates Gander like a store would.

How the zero-permission trick works

Gander receives files through the Storage Access Framework and “Open with” intents, so the OS hands it exactly the documents you chose and nothing else. Office formats render inside a locked-down WebView whose every request is intercepted by WebViewAssetLoader: bundled JS libraries load from app assets and the document streams from the content URI. No network stack is ever touched, and the app does not declare the INTERNET permission, so there is nothing to audit or trust.

Folder browsing uses ACTION_OPEN_DOCUMENT_TREE grants. Note that Android itself refuses to grant the Downloads root to any app; grant Documents, DCIM or a subfolder of Downloads instead.

Build from source

Requirements: JDK 17+ and the Android SDK (platform 35).

./gradlew assembleDebug        # installable debug build
./gradlew assembleRelease      # unsigned without a keystore

Release signing expects a local, untracked keystore at keystore/gander.jks (store and key password gander-local, alias gander); generate one with:

keytool -genkeypair -keystore keystore/gander.jks -alias gander \
  -keyalg RSA -keysize 2048 -validity 10000 \
  -storepass gander-local -keypass gander-local -dname "CN=Gander"

The keystore is gitignored on purpose: it is a personal signing key and must never land in a public repo.

Architecture in one paragraph

ViewerActivity routes by file extension first, MIME type second (FileKind.kt), into one of four surfaces: a native Pdfium view for PDF, a tiled SubsamplingScaleImageView for photos, Media3 ExoPlayer for video and audio, or a sandboxed WebView for everything rendered by vendored JS libraries (app/src/main/assets/viewer/). The home screen (MainActivity) lists recents (persisted SAF grants) and granted folders (DocumentsContract child queries), with thumbnails generated off-thread and cached (Thumbs.kt).

Vendored viewer libraries and their licenses: JSZip (MIT), docx-preview (Apache-2.0), SheetJS CE (Apache-2.0), PPTXjs + divs2slides (MIT), jQuery 1.11 (MIT), D3 3.x + NVD3 (BSD/Apache), marked (MIT), DOMPurify (Apache-2.0/MPL).

Roadmap

• F-Droid listing
• Legacy .doc / .ppt support if a usable offline renderer appears
• iOS companion (thin QuickLook wrapper)

Contributing

Issues and small PRs are welcome, see CONTRIBUTING.md. If Gander is useful to you, a star helps other people find it.

License

MIT. Vendored viewer libraries keep their own licenses, listed above; all are MIT/Apache/BSD and compatible.