CHM files are compiled Microsoft® HTML Help files suited for a wide range but mostly used as help files for applications. You can also use them inside the InfoPath task pane because they are URL accessible. There are many formats for the URLs as described in the link below. One of the formats is:
mk:@MSITStore:<path to CHM file>::<path inside CHM file>
You can enter an absolute path to the CHM file, for instance on a share or on the file system, but you can also include the CHM file as a resource in your form and access it by just using the filename as the path to the CHM file. The path inside the CHM file depends on the CHM file itself.
How to find the link from a CHM file
If you have an existing CHM file, you can easily find the hyperlink to a given page by right-clicking on the page and selecting properties. Note that the URL usually overflows and you can only see two lines, so to get the full URL you must click in the ‘Address (URL)’ field and select all text (Ctrl + A) and then copy-paste the link. For example the link to the first page in the Microsoft Office InfoPath 2003 help file is:
INFO: HTML Help URL Protocols
Microsoft HTML Help 1.4 SDK