Have a Mac 10.6.8 computer with Adobe reader 10.1.2 insllaled and in the past two days are no longer able to open a PDF file directly, we have repaired permissions on the computer, checked for updates in the application and verified that adobe is the default choice to open PDF's - The same issue has now started on a second computer
You could try changing the pdf ownership from Finder.
->Right click any pdf document
->Choose 'Get Info' from the context menu which appears
->In the file information dialog which opens, Choose 'Adobe Reader 10.1.2' in 'Open with' dropdown
->Click on button 'Change All'
Now try opening a pdf by double clicking. The pdf should open with Adobe Reader 10.1.2. Hope this helps.