CHM (Microsoft Compiled HTML Help) files are the oldest way to distribute help files on Microsoft Operating System. Though CHM files are widely distributed you can easily convert them to plain HTML files and view them on any operating system of you choice with a handy tool called CHM Decoder.
Related: How to print an entire CHM file?
CHM Decoder is a free decompiler for CHM files that will easily allow you to extract HTML files from a compiled CHM.
The process is simple, just open the CHM files you want to decode and click on the Go button to extract all the HTML files from the CHM. CHM Decoder is a freeware, so you can decompile CHM files without any restrictions.