CHM is windows based compiled Help file. These files would open only on windows or you may need some CHM view for Linux. Alternatively, you can convert .chm file to .pdf file using a tool called as 
chm2pdf application. Once they are converted to PDF you can view them using any PDF viewer.

Chm2pdf is a Python script that converts CHM files into
PDF files. The tools also support batch conversion, command line, PDF
security options, password protection, and compression modes while providing a Windows Explorer-like interface. It is also easy to install using the apt utility.<!>


This application can be installed using apt install command as follows.

sudo apt install chm2pdf


/usr/bin/chm2pdf [options] input_filename [output_filename]


Unstructured CHM files.

chm2pdf --webpage book.chm book.pdf

Structured Book CHM files

chm2pdf --book book.chm book.pdf