CHM files can be read using desktop applications, but that involves a lot of work: you have to download the software, setup, and then open the .CHM file. If you have too many CHM files, or if your everyday work involves you to read and edit CHM files, then you may want to download such applications.
But if you are going to read a single page, or only a couple of .CHM files, then I’d recommend you to use this addon for Firefox browser. A new addon called CHM Reader makes opening and reading the contents of the .CHM format files really easy. The addon is really small in size, and once downloaded, can be uninstalled quickly (you can always enable it).
Similar to Firefox’s PDF Reader which comes built-in with Firefox 15, this CHM Reader can open CHM files from the web. So, let’s say that you are searching for CHM documents online, and this can be opened right on your browser using the extension. I have downloaded the addon on my Windows, and here’s a screenshot of the reader: