KOReader is a document viewer for Linux based eink and generic Android devices. The formats it supports include Pdf, DjVu, XPS, CBZ, FB2, PDB, TXT, HTML, RTF, CHM, EPUB, DOC, MOBI, and ZIP files.
The first thing to get acquainted with KOReader would be to check out its
WiKi at Github.
KOReader has binaries/executable programs for: Kindle, Kobo, Pocketbook, and generic Android devices.
Please include which platform/device you are running KOReader on, in your posts here, especially if you are describing a problem.
(Brilliant intro to follow)
Luck;
Ken