To view CHM file in Ubuntu, you need to have a CHM viewer installed. GnoCHM is one of the CHM file viewer, designed to integrate nicely with Gnome, using PyCHM.
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| GnoCHM CHM Viewer | 
sudo apt-get install gnochm
The shortcut will appear in Applications > Accessories menu as "CHM Viewer".|  | 
| Access GnoCHM from the menu | 
This article discuss a simple way to view CHM file. If you know a better CHM viewer other than GnoCHM, please leave a comment.