Solution: How to open chm file?

Double-click to open the chm file.

You will get the following popped-up message box:
Frame title: Open File - Security Warning
Do you want to open this file?
Name: file1.chm
Publisher: Unknown Publisher
Type: Compiled HTML Help file
From: a path
[Open] [Cancel] buttons
[x] checkbox: Always ask before opening this file
Warning Icon: While files from the Internet can be useful, this file type can potentially harm your computer. If you do not trust the source, do not open this software.

Untick the checkbox 'Always ask before opening this file'.

Click on the Open button.

All text in the chm file is now viewable and not longer empty anymore.