Solved: I got the error message: You are on the wrong page. If you click on the top or bottom line with a number between 1 and 14, you will get this error message: Your item is already in use. It means that Acrobat has deleted your page or pages. So to delete a page, just right-click on the image of the page(s) and click on the Delete button. I used to get these errors when I try to use Quick look. That's because using Quick look, some pages and their thumbnails were created during the process that Quick look initiated. So any pages saved as an image would be deleted by Acrobat, and that is why it would fail to delete them. I have tried to delete my pages using several alternatives, such as Microsoft Word, and they all failed, and they didn't give me the errors when Acrobat's action was selected. But using this trick, my pages were successfully deleted by Acrobat DC, and they were back to normal! This trick might not work for some users. So I can only confirm if your page(s) are actually in use, and Acrobat is really deleting them! How to delete Adobe Acrobat PDF files manually? Try this Acrobat method : 1. First, open a PDF and zoom to 600% and save it to the hard disk as a .PDF file. 2. Zoom it, and save it at a new location, that you designate, and name it something like “PDF1.pdf”. 3. On Acrobat DC's home screen, type in “Save As” and select the default folder. You will then be prompted for the new name of your PDF files. 4. Then select the document, and choose the option “Copy”, and save it on your desktop. 5. Now right-click on the file and choose “Send to” (or “Save As”) and save the PDF file in Adobe Acrobat folder. 6. On Acrobat DC's home screen, select “File” (or “Open”), and choose the Adobe Acrobat folder. 7. Once you are there, double-click on the file, and you will see the PDF window. You can now close this window. 8. Restart Acrobat DC If you are still getting the same error when trying to delete pages, try some other options first.