I can't believe this is a seven year old question and that no one has yet mentioned the undisputed king of converters (and viewers), Calibre.
From the Faq:
What formats does calibre support conversion to/from?
calibre supports the conversion of many input formats to many output
formats. It can convert every input format in the following list, to
every output format.
Input Formats: AZW, AZW3, AZW4, CBZ, CBR, CB7, CBC, CHM, DJVU, DOCX,
EPUB, FB2, FBZ, HTML, HTMLZ, LIT, LRF, MOBI, ODT, PDF, PRC, PDB, PML,
RB, RTF, SNB, TCR, TXT, TXTZ
Output Formats: AZW3, EPUB, DOCX, FB2, HTMLZ, OEB, LIT, LRF, MOBI,
PDB, PMLZ, RB, PDF, RTF, SNB, TCR, TXT, TXTZ, ZIP
Calibre has been around since 2006, is open-source, stable, widely used (currently
(Active installs: 3,389,907 (those that have started calibre at least once in the last 60 days)), and well supported, inluding a tag on SuperUser, and runs on Linux, Windows, Android, iOSiOS andmacOS.
Check out the demo