OS X Application Window not showing or visible

I recently had a user trying to open Acrobat Pro DC on a Mac laptop. The user would get the menu bar across the top, but could not see the application window on the screen to actually do anything. The trick was to go to Window->Zoom, which brought the app window back to normal on the screen. Either it has been zoomed out so much it was too small to see, or it had somehow gotten moved off the screen to where it was open but not in an area the user could see it.