Microsoft Compiled HTML Help - Wikipedia
https://en.wikipedia.org/wiki/Microsoft_Compiled_HTML_Help
Microsoft Compiled HTML Help is a Microsoft proprietary online help format, consisting of a collection of HTML pages, an index and other navigation tools. The files are compressed and deployed in a binary format with the extension .CHM, for Compiled HTML.
CHM File Extension - What is a .chm file and how do I open it?
https://fileinfo.com/extension/chm
A CHM file contains help documentation compiled and saved in a compressed HTML format. It may include text, images, and hyperlinks. CHM files are used by Windows programs as an online help...