Opening CHM file

When I open a CHM file, sometimes the content of the file wont be able to display properly as shown below:

I found that we'll normally get this problem when we open a CHM file from a shared folder or new CHM downloaded from the internet.

To open the file properly is very simple. When you open the CHM file, if there is a "Open File - Security Warning" message box pop up, make sure you uncheck the "Always ask before opening this file" option.

I dont know why this happen though.