First, in order to start the OCR process in Acrobat DC (Windows), you must have a PDF file open. If you want to OCR multiple files, whether they're open or must be added from a folder, you must choose the Add Files option at the top of the window. That's when you get the "menu" options "Add files", "Add folders" and "Add open folders," which is all fine. But the initial file has already been selected and has populated the window, so if you "Add Open Files", for instance, you always get the annoying pop-up error that the first file is already open--unless you go through some time-consuming gyrations to make sure that file is unselected then reselected. But you do want that initial file to be scanned, otherwise you wouldn't have picked it in the first place. If you want to show the user the error--at least that particular error--put a text box on the selection window and display it. Don't stop the whole process with a message box and make the user verify that, yes, that's really what they want done. While I'm at it, a recent update to Acrobat has reversed the Cancel and OK buttons on that window, which is different than every other window in Acrobat and definitely does not conform to the Windows standards for GUIs--at least the older ones that I'm familiar with.