If you are trying to mark something sensitive out, make sure that you are deleting the actual text or convert the PDF to a flat image after. PDFs can store information in text and images, so if you just draw over some text thinking you are marking it out, there is a chance that the mark out is just a image layer sitting on top of the sensitive text. A way to check this is opening the PDF in Firefox after and toggle Reader View (button in the address bar or F9) to see if you can still see to marked out information.