Nothing happens if I hit the delete key, and the delete option is greyed out in the Edit menu. I tried it with many different PDFs and it doesn't work, but I only just found one on which it does work. It happens to be a PDF I've created myself. The other ones are articles I've downloaded from the internet. But they are not protected. I've tried to copy or duplicate them already, but the problem persisted. Okay I just tried to export the existing PDF (from the internet) to another PDF (I'm not sure how this works because I think when you export or save as, it only does that for the selected page of the PDF) but then I noticed that it IS a protected document: it asked me a password in order to save the document somewhere else
that's weird because in the Information window of that file, it isn't marked as protected.