Why is it that I can't edit some pdf music sheets. It works in the majority of cases but in some instences the editing features just simply don't work, I can't write, I can't highlight. I can create a text box, input text, but as soon as I step out of the box, the text disappears. My only work around is to print the pages, rescan them, upload them again and that usually works I can then use the features, but that's a waste of time and paper. Any clues?