Users on some version of Windows will find that they might be blocked from opening/using the CHM help file. To avoid the main problem, always save the file to disk and open from there rather than directly from the web page.
Even after that though, once the file is saved to disc, double-clicking it may result in an Open File - Security Warning dialogue asking the user, Do you want to open this file?. Clicking Open will open the Help dialogue but the user may see a message similar to the following in place of the content for every topic:

Navigation to the webpage was cancelled

To fix this issue the user must:

• Right-mouse click the file and select Properties.
• On the General tab press the Unblock button.
• Click OK on the file properties dialogue.

Now, double-clicking the file should allow the user to see all of the html topics as expected.