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
You can turn on the watermark feature by doing: “Edit a PDF file”, then “PDF Settings” at the bottom of the window. In the Privacy section, click Watermarks, click Enabled. If you want to remove the watermarks, you can do: “View a PDF file”, then “PDF Settings” in the section “Watermarks” and select “Remove Watermark”. There should now be no watermarks on any pages in the file. You can also turn on watermarks for more than one document by selecting multiple files and doing “View a PDF file”. To turn off watermarks, click to the “Settings” section for the watermarks and to turn off them (all) for any file select “View a PDF file” then “Settings” for the watermarks and set the value of None to “Apply to all files” and “Allow a document to have Watermarks” to “Show me watermarks when I open.