Microsoft Compiled HTML Help or more commonly known as .CHM format, is a proprietary format for online help files by Microsoft which was first released in 1997 in Windows 98 as a successor to the Microsoft WinHelp format. CHM packages and archives all HTML files into one single file, including the associated images and other media.
Although been replaced with new generation of Windows Help called Microsoft Assistance Markup Language since Windows Vista, .CHM frmat is still supported and distributed through Windows XP and Vista platform (although HLP extension format has been dropped in Vista). However, Linux and Mac OS X machines do not have built-in support for CHM format either, and requires user install third party tools for CHM support.

There are plenty of CHM files viewer libraries for Linux operating system that allows Linux users to read and open CHM files. And most of the CHM viewer libraries is available in the default distribution repositories of popular mainstream LInux distros, or in ports collection of BSD system.

libchm and chmlib are a very basic but sufficient library for dealing with Microsoft ITSS/CHM format files. These two libraries (each for different Linux distribution) serve to provide basic functionality related CHM files, which other CHM viewer tools rely on and use. With just the basic library, user will be able to extract the contents of the CHM files. And as most third-party CHM tools to view CHM on desktop require the basic library too, it should be installed first, if it’s not installed yet.

Ordinary, the opensource chm viewer have KchmViewer 、 Help Explorer Viewer 、 ChmSee 、 GnoCHM 、 xCHM、 KCHM and so on. Otherwise, you can use CHM Reader Add-On Extension in Firefox browser.