That is odd, indeed. Can you tell us any other details about this? Is this Reader or Acrobat? If Acrobat, can the file be opened in Reader? I'd recommend a Repair Installation in any case - it generally can help, but not hurt many situations. Also check to be sure you have the latest version installed.
As described, you are able to open up PDFs fine via Adobe on the admin user on the 3rd party application. It seems to be some issue with the user profile. Please try to repair the user profile and see if that helps.
Also please check out the help pages listed below: