tl;dr: Gander is a small (~15MB) open-source Android file viewer that renders PDFs, Office documents, images, video, audio, Markdown, and code entirely on-device, without requesting any permissions—including INTERNET. It uses the Storage Access Framework and "Open with" intents to receive files, with Office formats rendered in a locked-down WebView using bundled JS libraries (docx-preview, SheetJS, PPTXjs). Available as a sideloaded APK or via Obtainium; legacy .doc/.ppt aren't supported due to lack of a viable offline renderer.