OK, I think this is an interesing situation.
I have changed the program association for PDF files in Windows from Adobe reader to a PDF editor that we purchased. From Windows Explorer (in XP and/or 7) I can double click a PDF file and it will open in the editor which is correct based on my file associations.
In Access (2003 and/or 2010) I have a couple of image buttons and links that I set the path (HyperlinkAddress) and file name to the appropriate PDF file if it exists. When I click on the links, the Adobe Reader opens, not the editor.
I am not specifying (in Access) what program to use to open the file yet it is using a program that is no longer associated with the extension. I should be able to use the associations since some users will be using the Adobe Reader and some the new Editor.