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MuPDF - MuPDF is a lightweight PDF and XPS viewer.
calibre - Ebook manager, viewer & converter
Okular - Okular is a universal document viewer based developed by KDE.
FBReader - FBReader is an e-book reader for various platforms. Features:
qpdfview - qpdfview is a Qt-based tabbed document viewer using the Poppler library for PDF rendering and CUPS...
Google Play Books - Google Play Books lets you search, preview, and buy millions of books using Google Book Search.