viewers
Desktop
Desktop standalone applications:
To open a .chm file with linux, use one of the following viewers:
app-text/kchmviewer
      Homepage:      http://www.kchmviewer.net/
      Description:   A feature rich chm file viewer, based on Qt
app-text/xchm
      Homepage:      http://xchm.sourceforge.net/
      Description:   Utility for viewing Compiled HTML Help (CHM) files
Plugins
ChmFox brings the best CHM file reading experience to Firefox
https://addons.mozilla.org/en-US/firefox/addon/chmfox/
Convertors