You can link from one CHM help file to another one's specific topic as follows:

  • Make sure both CHM files are placed in the same folder
  • In your HelpNDoc project, create an Internet link with the following URL (without the quotes): "ms-its:YOUR_SECOND_HELP_FILE.chm::/HELP_ID.htm"

Where you replace "YOUR_SECOND_HELP_FILE" by the file name of the other help file and "HELP_ID" with the Help ID of the topic to link to.

You can learn how to setup Help IDs for your topics in the following step by step guide:

You can learn more about this topic from Microsoft's own documentation: