Write a waved watermark across every page of a PDF and burn it into the paper.
Type a word and it is repeated across every page: leaning, following a wave, in rows of grey and pink under a soft shadow. It is then burned into the page rather than laid on top, so nobody can select it and delete it - which is what happens to a watermark that is only text - and the pixels beneath it are coarsened, so it cannot be subtracted back off either. The trade is that the pages become pictures, so the document stops being selectable and searchable too. A watermark marks a document as yours; to keep one private, use Password Protect.