How to decode a .chm file?
This is pretty easy to do for free you just have to use Terminal and install Homebrew and chmlib.


Open Terminal. (/Applications/Utilities/)
Install Homebrew by visiting http://brew.sh/ and follow the installation instructions. Or just copy paste this command into Terminal and run it. 

/usr/bin/ruby -e "$(curl -fsSL https://raw.githubusercontent.com/Homebrew/install/master/install)"

After the install is complete type brew install chmlib in Terminal and press enter.
Move the file you want to extract to your Desktop.
Go back to Terminal and type:

extract_chmLib ~/Desktop/name_of_the_file.chm ~/Desktop/name_of_the_file


Just be sure to replace name_of_the_file.chm with the actual name of the file. This will create a new folder on your desktop.
Open the folder and double click the file index.html. This will open the document in your web browser.