CHM (Microsoft Compiled HTML Help) is a Microsoft proprietary online help format, that consists of a collection of HTML pages and an index.
CHM files are not recognized by Linux and they cannot be open using any default application.
To open CHM files, you have to install CHM viewer (like xCHM) in Linux.
You can use xCHM to open CHM files in Linux.
xCHM is an open source and free software which compiles and runs on Linux, BSD, Mac OS X and Microsoft Windows.
In Ubuntu Linux, you can install xCHM with the following command:-
sudo apt-get install xchm
After you install xCHM, you can open any CHM file by right clicking it and selecting ‘Open with xCHM‘ menu.
Hope it helps. Thanks.