calibre - Ebook manager, viewer & converter
DocSlide - DocSlide is an online document and knowledge sharing platform.
FBReader - FBReader is an e-book reader for various platforms. Features:
SlideShare - Discover, Share, and Present presentations and infographics with the world’s largest professional content sharing community.
Google Play Books - Google Play Books lets you search, preview, and buy millions of books using Google Book Search.
Scribd - Unlimited books, audiobooks & comics. Unparalleled discovery. Any device. $8.99/month