CHM files have largely been superseded by HTML in the A-Shell library. CHM files were the best choice in days of limited internet access—and still are the best choice when the internet is not available. Here is some general information on CHM files.
Background Information
● Compiled HTML help ("CHM") is a proprietary format for help files developed by Microsoft. It was first used in 1997 (Win98) as a successor to the even-older WinHelp format, and is still supported through Windows XP, Vista, Win7 and Win8. Microsoft no longer creates CHM files for its own documentation and has moved on to a new format, but CHM will likely be supported for a long time.
● CHM files consist of individual web pages plus a hyperlinked table of contents. The resulting file is compressed using LZX compression, making CHM files rather efficient and relatively small.
● CHM files are created using a free tool from Microsoft called "HMTL Workshop." Many additional tools for creating CHM files, as well as other help-file formats, are on the market. They are collectively known as "help authoring" tools or systems. The help file you're reading right now was created by a combination of Microsoft Word and a help-authoring tool called "Doc to Help."
● CHM files contain "active content"--i.e., the files are executable—and therefore are frequently perceived as being security risks. Gmail, for example, blocks CHM files.
● For more information, see the Wikipedia entry for CHM files (from which some of the above facts were taken).
Problems and Solutions
● For security reasons, CHM files will normally not be displayed properly or at all when you are trying to access them from network drives or remote servers. It is possible to change your security settings to accommodate such files, but the easier solution is just to move the CHM file(s) to a local drive.
● CHM files will not work if their path or filename includes the pound (aka hash) character '#'. The hash character signifies an anchor in HTML so the CHM viewer fails to resolve the path properly and to retrieve the content. If this is your situation, move the file to a path that does not have a hash character in its name. In general, avoid using '#', '?', '&', '+', and other special characters in directory names.
● When you click on an external link inside a CHM file, the web page is opened with Internet Explorer and not with your computer's default browser. Why? Since the CHM viewer itself is a component of IE, or *is* IE operating in some special mode, the program does not issue a conventional call to the PC to use the default browser. Instead, since IE is already running and available, IE simply "tells itself" (?!!) to open the external page. There is no work-around to this "problem."
● Windows (XP/Vista/Seven) security provisions may prevent CHM files from opening. If you get a "cannot display" message in the right pane of the display window, this may be the problem. Solution: right-click on the CHM file and select Properties from the popup menu. Click on the Unblock button immediately below the Advanced button on the General page, then click Apply. Once the CHM file has been unblocked, the Unblock button disappears, and you need only do this once—i.e., not every time you open the file.
● If the ideas and suggestions mentioned above do not resolve your problems opening CHM files, you might want to do a web search for "MJ's Help Diagnostics." This is a program that checks to see if the Windows programs and components needed to display CHMs are properly and completely installed on your (Windows) computer.