To force Windows to use a particular extension

To force windows to use a particular program, the most effective way is as follows. That will show you the program currently associated with PDF files.
 * Open a command prompt with administrator privileges.
 * type: assoc .pdf

Example
to set it to SumatraPDF type the following.
 * type: assoc .pdf=SumatraPDF

I used this to associate .epub, .cbr, .pdf with Sumatra and it worked when all the other methods failed.

You can use this method to associate any file with any program

To see ALL associations


 * type: assoc|more