FastStone Image Viewer vs Mozilla PDF.js

FastStone Image Viewer is a Windows image browser, converter, and editor for individual images and collections. Editing tools include resizing, rotation, cropping, color adjustment, sharpening, blur, Clone Stamp, and Healing Brush. It supports major graphic formats and popular camera RAW formats, including HEIC, WEBP, PDF, SVG, and JPEG XL. The Compare Images feature displays up to four pictures side by side, while batch tools convert and rename collections. Slideshows offer more than 150 transition effects and music. The program can acquire images from scanners and batch scan to PDF, TIFF, JPEG, or PNG. Users can launch external editors from within Image Viewer, and a portable version can run from removable storage. FastStone lists personal and educational use as free and a commercial lifetime license at $34.95 as a one-time purchase, with upgrades free. Another plan listing gives a price of $34.95 a month, so the commercial license terms should be confirmed before purchase. The product is listed for Windows, and FastStone provides technical support by email.

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.