Having certain apps installed might prevent PDF Viewer from opening or creating documents. Before proceeding understand this could lead to data loss so please be sure to follow each step carefully and start by backing up the device. A properly made backup will ensure you can restore back to your original state.
Follow these steps to pin-point which app is causing the problem:
Very Important: Complete a back-up of the iPad before proceeding
Do a complete factory reset of the iPad
After reset is complete re-install PDF Viewer and test that its working as expected
Begin re-installing your other apps that have access to files with the .pdf extension
Test PDF Viewer after each app is installed until PDF Viewer Pro fails like you are currently experiencing (this shows the last installed app is causing PDF Viewer to misbehave).
Now that you have identified which app is causing the problem you can use the backup you created to restore the iPad back to its original state before testing. Once that is done completely uninstall the app you identified during testing that interfered with PDF Viewer. I also strongly suggest reporting the app to us through the Support link found in Settings so we can investigate correcting the behavior.