CHM is a file format for Microsoft Compiled HTML Help, a format of compiled and compressed heavily indexed HTML pages.

It is a Microsoft’s proprietary format, but there programs available in Linux that can read the files with the .chm extenxion. Though there are few different viewers in Linux, they are based on the same library, the libchm.

These 3 are the mostly used viewer for Linux;

• xCHM
• GnoCHM (Gnome / GTK based)
• KchmViewer (Kde / QT based. Latest version available for KDE 4)

To install them all in Ubuntu;

sudo apt-get install xchm gnochm kchmviewer