Okular - Okular is a universal document viewer based developed by KDE.
Documents by Readdle - A central hub for all your files.
Sumatra PDF - Sumatra PDF is a slim PDF/DjVu/EPUB/XPS/CHM/CBR/CBZ/MOBI viewer for Windows.
GoodReader - With GoodReader on your iPhone, iPod Touch or iPad, you can read virtually anything, anywhere.
Foxit Reader - Foxit Reader is a free and light-weight multi-platform PDF document viewer.
FileApp - FileApp is an iPhone and iPad File System and Document Viewer.