Hi Tina!
Why are you selling them as PDFs? Do they need to be resizable to any size? You could try saving the PDF as a read only file (after saving, right click on the file, go to properties, check "Read only") but I'm not sure if that'll prevent it from being edited in Illustrator. Another alternative would be to save your digital downloads as VERY large PNGs so that they're mostly resizable.
Good luck!