The files are compressed and deployed in a binary format with the extension .CHM, for Compiled HTML.
[7] Although the format was designed by Microsoft, it has been successfully reverse-engineered and is now supported in many document viewer applications.
Microsoft has announced that they do not intend to add any new features to HTML Help.
[13] Sumatra PDF supports viewing CHM documents since version 1.9.
[14] The Free Pascal project has a compiler (chmcmd) that can create CHM files in a multiplatform way.