Sumatra PDF - Sumatra PDF is a slim PDF/DjVu/EPUB/XPS/CHM/CBR/CBZ/MOBI viewer for Windows.
Documents by Readdle - A central hub for all your files.
calibre - Ebook manager, viewer & converter
FE File Explorer - The one-stop files solution
Okular - Okular is a universal document viewer based developed by KDE.
FileApp - FileApp is an iPhone and iPad File System and Document Viewer.