Mac OS wont open a file if the App is open - Need to quit the app every time to open files
There's a few things to try here to help us narrow down this problem.

Boot into Safe Mode

Follow these steps to boot your Mac into Safe Mode:


Fully shut down your Mac
Restart your Mac
Immediately press the Shift key and keep it down 
Let go of the Shift key when you see the login window 
Now log into your Mac and (NOTE: If you have FileVault enabled you may need to log in twice)
Now test to see if you have the same problem
Exit Safe Mode by restarting your Mac as normal
Once again, test to see if you have the same problem


Once you've done this let me know how you went and we'll go from there.