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.
HelpSmith - HelpSmith is a help authoring tool to create CHM HTML help files, manuals, and PDF documents.
Foxit Reader - Foxit Reader is a free and light-weight multi-platform PDF document viewer.
Adobe RoboHelp - Adobe RoboHelp empowers organizations to deliver engaging help and policy content across mobile devices, formats, and platforms.