What are some alternatives?
When comparing FBReader and Sumatra PDF, you can also consider the following products
calibre - Ebook manager, viewer & converter
Foxit Reader - Foxit Reader is a free and light-weight multi-platform PDF document viewer.
Amazon Kindle - Amazon Kindle software lets you read ebooks on your Kindle, iPhone, iPad, PC, Mac, BlackBerry, and...
PDF Reader X - PDF Reader X is a nest in the class PDF reader, PDF annotator and PDF editor for the Mac OS X operating systems.
Cool Reader - Fast and small cross-platform eBook reader for desktops and handheld devices
Evince - Evince is a document viewer for multiple document formats: PDF, Postscript, djvu, tiff, dvi, XPS...