Acrobat and Apple Mission Control drag and drop
If Acrobat is open, and you use hot corners in Apple Mission Control for "Show Desktop", icons on the desktop are not selectable. If Acrobat is closes, normal Mission Control behaviour resumes.
Also, if this helps isolate the issue, it seems to be related to a small ultra minimised window that appears occasionally (not always) that seems to be associated with Acrobat.
Screen shot: https://adobe.ly/2nqnUY1
This appear in the bottom left corner. If I maximise to fullscreen it is blank, if I close it there doesn't appear to be any ill effect, but suddenly I can drag and drop while in mission control again.
The issue persists outside of fullscreen mode. I have made a video demonstrating it, and linked it here: https://adobe.ly/2EsjTK4
AdminAnkit (Admin, Adobe) commented
I believe that you are using Acrobat in full screen mode. If yes, then I would like to tell you that hot corners do not work when application is used in full screen mode.
Though, if you are not in full screen mode and still hot corners are not working for you, please do let us know.