coolreader (e-book reader)

CoolReader is an e-book reader for various platforms, with a Qt5 user
interface.

Supported file formats include plain text, HTML, RTF, Microsoft Word
(.doc), Palm Pilot (.prc, .pdb, .pml, .mobi), FB2, and EPUB.

Note: coolreader is "chatty": it logs a lot of meaningless warnings
and errors to stderr. Unless there's an actual problem with the
application, you can ignore this stuff.