calibre - Ebook manager, viewer & converter
Okular - Okular is a universal document viewer based developed by KDE.
ReadEra - Read book files in EPUB, PDF, DOC, DOCX, RTF, TXT, DJVU, FB2, MOBI and CHM formats.
Sumatra PDF - Sumatra PDF is a slim PDF/DjVu/EPUB/XPS/CHM/CBR/CBZ/MOBI viewer for Windows.
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.