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. The format is often used for software documentation.
CHM was introduced as the successor to Microsoft WinHelp with the release of Windows 98 and is still supported in Windows 7. Although the format was designed by Microsoft, it has been successfully reverse-engineered and is now supported in many document viewer applications.
You can convert PDF to CHM by PDFConvert desktop software or command line program.