Librera Reader vs Mozilla PDF.js

Librera Reader is an ebook and document reader for Android, with a browser-based reader and a macOS beta also listed. It opens formats including PDF, EPUB, MOBI, DjVu, FB2, TXT, RTF, AZW, AZW3, HTML, CBZ, and CBR. Readers can locate files by scanning folders, browsing with the in-app file explorer, or returning to recent items. Reading tools include bookmarks, annotations, text search, EPUB3 tables of contents, and adjustable Day and Night modes. Progress and bookmarks can sync between Android devices through Google Drive. The app also offers configurable text-to-speech with a built-in media player, online catalogs, cloud sources, and online and offline dictionaries. Access can be restricted with a password or the device’s fingerprint authentication. The free plan is ad-supported; Librera PRO removes ads and costs 5.99 USD once. The maker lists Kyiv, Ukraine as its headquarters.

Mozilla PDF.js is a free, web standards-based platform for parsing and rendering PDF files. Developers can use its display API to render documents and retrieve information, or adapt the viewer layer for a custom interface. That viewer is used in Firefox and project browser extensions. PDF.js renders annotations and supports adding a subset of annotation types, and it supports form filling; OCR is not provided. The project targets HTML5-compliant browsers and includes a legacy build for older browsers, although PDF feature support varies by browser and version. Prebuilt builds are available, with hosted copies listed through jsDelivr, cdnjs, and unpkg. Developers can clone and build it locally, but some browsers require a local web server instead of opening PDFs through file URLs. Cross-origin requests are blocked by default, so CORS or a server proxy may be needed. For large documents, rendering only visible pages can reduce memory use. PDF.js is licensed under Apache 2.0.