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gander

Introduction: Take a gander at any file. Offline, zero-permission Android viewer for PDF, Word, Excel, PowerPoint, photos, video, audio, Markdown and code.

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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.

Gander demo: thumbnail recents, folder browsing, PDF, Word, Excel and Markdown viewing

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

Category	Formats	Renderer
Documents	PDF	Pdfium (native)
	Word `.docx`	docx-preview, offline in a sandboxed WebView
Spreadsheets	`.xlsx` `.xls` `.xlsm` `.xlsb` `.csv` `.ods`	SheetJS, offline
Slides	PowerPoint `.pptx`	PPTXjs, offline
Photos	JPG, PNG, WebP, BMP, HEIC/HEIF	Tiled deep-zoom image view, EXIF aware
	GIF (animated), SVG, AVIF, ICO	WebView
Video	MP4, M4V, MOV, MKV, WebM, 3GP, AVI, FLV, MPEG-TS	Media3 ExoPlayer
Audio	MP3, M4A, AAC, FLAC, WAV, OGG, Opus, AMR	Media3 ExoPlayer
Markdown	`.md` rendered as formatted HTML	marked + DOMPurify, offline
Text and code	`.txt` `.json` `.xml` logs, most source files	Text viewer

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

Runs on Android 8.0 (API 26) and up.

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).
Copy it to your phone, tap it, and allow "install unknown apps" when asked.
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.

Verify before installing: every release is signed with the same key, so you can confirm an APK really came from this repo. Obtainium can pin the fingerprint below, and for a file you have already downloaded:

apksigner verify --print-certs Gander-x.y-arm64.apk

Signing certificate SHA-256:

5B:5C:F6:4A:94:23:7C:D5:F0:E0:85:76:00:38:BC:1C:EB:DF:18:DA:BA:5C:B3:EA:CA:7C:15:9F:22:A7:E2:4B

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

To build it yourself you need JDK 17+ and the Android SDK (platform 35). These are build requirements only. The installed app runs on Android 8.0 (API 26) and up.

./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).