The issue is likely to be caused by one of the following:
Adobe is not installed on your PC. This can be resolved by installing Adobe.
File association is relating to another instance of Proclaim for the .pdf file.
To resolve the issue of file association, please follow the steps below:
On your PC search for Control Panel and open it.
Select Default Programs.
Then select Associate a File Type or Protocol with a Program.
Scroll down and in right hand column choose Default Apps by File Type.
In left hand column scroll down to .pdf and look in right hand column to see what the default program is. It may say Chrome or something else, but not Adobe. Click on the app on the right of the screen and select Adobe Reader.
Log out of Proclaim and in then log back in again. The .pdf files should now open. If they do not then reboot the PC and try again so the changes are picked up by Windows and your Proclaim version.