CHM files are so-called compiled Html Help files or Microsoft Compressed HTML Help, so put together help data in html format, which were introduced by Microsoft with Windows 98 and are still used today for many Windows help texts.
On Windows, CHM files can be displayed easily, on other systems, a display program is usually necessary. Some CHM files can also be viewed with a browser, but this depends on the structure of the CHM. If you only want the individual files, you can also open a CHM file with an unpacker like 7-zip.
For example, CHM files can be created using the free Microsoft HTMLHelp Workshop. There are also converters that can create CHM files from pdf or Word files.
Most often resolving problems with opening CHM files is very simple. Just install an appropriate program that supports such files. All of the listed programs support CHM files, but may vary in offered function and purpose. Some programs may be capable only of viewing contents of CHM files or offer file conversion options, but may not be capable of editing such files.
It is possible that although a compatible program has been installed on user’s system, it is not used by the system to open it. This may happen because no default application was associated with this type of files. To associate CHM files with given application, select "Open with" from drop-down menu (accessed by right-clicking on the file). Then select from the list the program or application you want to be used to open this type of file. If the xCHM applications is not on the list, select "Browse" option in order to manually locate the directory where xCHM has been installed.
Sometimes problems with opening CHM files may not lie with the application itself, but can arise due to other causes, such as: