What are some alternatives?
When comparing Amazon Kindle and Sumatra PDF, you can also consider the following products
FBReader - FBReader is an e-book reader for various platforms. Features:
Foxit Reader - Foxit Reader is a free and light-weight multi-platform PDF document viewer.
calibre - Ebook manager, viewer & converter
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
Okular - Okular is a universal document viewer based developed by KDE.