How to open chm file in Windows?

For many years, I cannot open CHM file in Windows. I never knew why. I thought, the CHM file was old or outdated or generated with old compiler or wrong encoding.

Until one day I am so fed up, I search for solution. Surprisingly, there are some softwares offer to open CHM file, including a plug-in in Firefox to open CHM file. However, the Firefox plugin still cannot open some CHM file correctly, there are missing pages or images.

Finally, I come across this simple solution:

I read somewhere, for security reason, Windows cannot open downloaded CHM file automatically. We have to right click the file -> Properties -> General tab -> [Unblock] button.