Filza File Manager - Explore files and folders with list view or grid view. View / edit files with powerful viewers.
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.
FE File Explorer - The one-stop files solution
Foxit Reader - Foxit Reader is a free and light-weight multi-platform PDF document viewer.