Building Nexo Player: An Offline-First Android Media App with PDF-to-Audiobook Support

Most Android media apps solve only one part of the problem. A video player plays videos. A music player handles songs. A PDF reader displays documents. A text-to-speech app reads text. A vault hides private files. But real media libraries are not separated that neatly. My phone may contain downloaded movies, music, lecture notes, ebooks, PDFs, recordings, subtitles, and files I do not want exposed in the normal gallery. Constantly moving between different apps creates friction and breaks playback or reading continuity. That is why I built Nexo Player : an offline-first Android media app that brings local playback, document reading, audiobook generation, text-to-speech, and private storage into one experience. What Nexo Player does Nexo Player currently supports: Local video and audio playback PDF and EPUB reading PDF, EPUB, and text narration Background audiobook generation MP3, M4B, and ZIP export Multiple narrator voices Resume playback and reading progress Equalizer, sleep timer, subtitles, and playback-speed controls Picture-in-Picture Secure Vault protected with PIN or biometrics The app is built natively for Android using Kotlin , Jetpack Compose , and Android Media3 . The main product idea: local-first media The core rule behind the app is simple: A local file should remain local unless the user explicitly chooses otherwise. This rule influenced the entire product. Opening a downloaded video should not require an account. Reading a PDF should not require uploading it to a server. Listening to a generated audiobook should remain possible without a permanent internet connection. Private files should not leak into normal galleries, thumbnails, or recent-history screens. Offline-first is not only about caching data. It means the main workflow must remain useful, understandable, and recoverable without depending on the network. Building the playback layer For video and audio playback, I used Android Media3 as the foundation. The visible player looks simple, but a