Okular - Okular is a universal document viewer based developed by KDE.
HelpNDoc - What is HelpNDoc ? Easy to use yet powerful help authoring environment which can generate various documentation formats from a single source.
Sumatra PDF - Sumatra PDF is a slim PDF/DjVu/EPUB/XPS/CHM/CBR/CBZ/MOBI viewer for Windows.
Dr.Explain - Dr.Explain software is a help file authoring tool.
Foxit Reader - Foxit Reader is a free and light-weight multi-platform PDF document viewer.
MadCap Flare - Documentation for Any Audience, Language or Format