Even if you work only in Linux, you’ll likely have to use Microsoft Compiled HTML Help (CHM) files at one time or another. Several open source projects use this common format, including Apache, MySQL, PostgreSQL, Python, and PHP. Microsoft developed CHM as a proprietary format for Windows 98, leaving behind the previous WinHelp (HLP) format. CHM is still alive and kicking in XP and Vista, though some applications use the newer Microsoft Help 2 format.
Six CHM viewers for Linux