GoodReader!

What is Sumatra PDF?

Calibre is a free and open source e-book library management application made by users, for users. Many languages, designed around the concept of the logical book, i.e., a single entry in your library that may correspond to actual e-book files in several formats.

Foxit Reader is a free and light-weight multi-platform PDF document viewer. Its core function is compatible with PDF Standard 1.7.

Evince is a document viewer for multiple document formats. It currently supports pdf, postscript, djvu, tiff, dvi, cbr, cbz, etc.

Xodo is the ultimate cross-platform PDF viewer and annotator. And it’s 100% free.

A fork of the last GPL version of EBookDroid. Supports the following formats:

Notability is the first truly integrated note-taking app for iOS. Standing on a framework that automatically links notes with audio recordings, Notability supports all of your note-taking needs -- handwriting, PDF annotation, word processing, and more work together seamlessly allowing you to create comprehensive, beautiful notes, quickly and simply. Auto Sync ensures your notes are backed up safely. And now you can personalize the look and feel of Notability’s library and toolbars with Themes, from colorful, to clean, to classic. »

PDF Expert lets you read and annotate PDF documents, highlight text, make notes and save these changes being compatible with Preview and Adobe Acrobat. Moreover, PDF Expert is the only iPhone application that can fill PDF forms!

Besides exceptionally fast launching and displaying speed, Gaaiho Reader is reliable in printing, filling forms annotating documents and adding bookmarks.

qAllInOne (aka qAIO) is a free open-source software built using Qt to provide users an All-In-One media player, so it can view images, play videos and audio.

Read, search, make notes & highlights in PDF (Adobe DRM), EPUB (Adobe DRM), DjVu, TXT, FB2, FB2.zip book formats. Enjoy a unique, fast and user-friendly interface that was designed to provide the most convenient set of options for you to fully enjoy reading a book.PocketBook reader allows you to customize your reading experience by adjusting text size, layouts, colors and many other options.

Atril is a simple multi-page document viewer. It can display and print PostScript (PS), Encapsulated PostScript (EPS), DJVU, DVI, XPS and Portable Document Format (PDF) files. When supported by the document, it also allows searching for text, copying text to the clipboard, hypertext navigation, and table-of-contents bookmarks. Atril is a fork of Evince.

Freebook Sifter lists free eBooks from various sources. They update the list daily and you can find free eBooks from Amazon sites around the world, Kobo, Nook, Project Gutenberg.