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Pro Free and open source
Sumatra is available for free, licensed under GPL with source code available on GitHub.
Pro Reads many file types
Including PDF, XPS, DjVu, CHM, EPUB, Mobi, FictionBook, PalmDoc, and comic books.
Pro Book updates on the fly
If Book font, style is changed, Sumatra will take the update instantly, No need to close first and import again! Huge time saver for self publishing.
The application consists of a single file that can be run without installation. You can put it on an external device, such as an usb thumb stick and run it on any Windows machine from the thumb stick.
It's available in 69 languages.
Con Windows only
No Linux or OSX versions available.