I was working with a customer who uses QuickBooks, and they stopped being able to view PDFs saved in QuickBooks. I tried a few things and even got QuickBooks support on the line to help troubleshoot. They were unable to resolve the issue, but the rep did give me a link to a KB. The link stated that Adobe Reader DC was not supported by QuickBooks and recommended going to Adobe Reader 11.
When QuickBooks does not find a compatible PDF reader it defaults to IE, and IE sees the \\ as a URL and tries to open a nonexistent site. So the solution is to downgrade to Adobe Reader 11. Then you have to make sure that the file you open in QuickBooks is not still pulling the UNC name. You can check this by pressing f2 and looking at the "Location" it should not start with \\. If it does, then you need to close out of QuickBooks, map a network drive to the shared folder that the QuickBooks files reside ie...\\DC1\QBData for instance. Reopen QuickBooks and open the file from the mapped drive you just created. When you press f2 now, the "Location" should say N:\DC1\QBData (or whatever your mapped drive is).
Here is a link to a KB providing a step by step - https://community.intuit.com/articles/1437250