Microsoft Compiled HTML Help (CHM) is a Microsoft proprietary online help format. It was introduced as the successor to Microsoft WinHelp with the release of Windows 98, and is still supported in Windows 7.
Help is delivered as a binary file with the .chm extension. It contains a set of HTML files, a hyperlinked table of contents, and an index file.
The file starts with bytes "ITSF" (in ASCII), for "Info-Tech Storage Format". The format has been reverse-engineered by Matthew Russotto with assistance from Peter Ferrie and Paul Wise. Russotto's documentation is freely available at http://www.russotto.net/chm/chmformat.html.
CHM file is not recognize by Linux and cannot be open using any default application, you can either use chm viewer or convert this chm file to some other format like PDF to view