Watermark On Pdf can add watermarks to all or some pages of a PDF file. If you then secure the document with a password to prevent changes, it can't be removed
On the other hand, if you do nothing and the document doesn't change, the watermark won't appear. The other problem is that watermarks can't be added once a PDF file has been saved. There is an add-on that can convert text documents to PDF's, but this is only for text. Watermarks can't be added.