Are you sure your PDF file type is properly set up in your Windows system?
Double clicking on a pdf file in windows explorer should usually confirm.
Installing multiple pdf readers can sometimes cause problems:-
It might be worth removing any/all pdf reader software and installing/reinstalling the latest
version of Acrobat Reader if you are in any doubt.
TheBrain 18.104.22.168 has exhibited strange behavior with regard to matching correct icons versus file types.
I have yet to experience files not opening correctly though even if the icons are displayed incorrectly.
This suggested fix re this area may possibly help too:-
If the above fails or you are not confident tinkering with settings I suggest emailing
Windows XP Professional 32bit