What are some alternatives?
When comparing Amazon Kindle and Okular, you can also consider the following products
FBReader - FBReader is an e-book reader for various platforms. Features:
Sumatra PDF - Sumatra PDF is a slim PDF/DjVu/EPUB/XPS/CHM/CBR/CBZ/MOBI viewer for Windows.
calibre - Ebook manager, viewer & converter
Evince - Evince is a document viewer for multiple document formats: PDF, Postscript, djvu, tiff, dvi, XPS...
Cool Reader - Fast and small cross-platform eBook reader for desktops and handheld devices
Foxit Reader - Foxit Reader is a free and light-weight multi-platform PDF document viewer.