EDDiMark PDF is an Easy Digital Downloads extension that adds a watermark to pages of your PDF file(s). The watermark is customizable with font face, font color, font size, placement, and text. Not only that, but since the watermark is added when a file download button is clicked (either on the customer’s order confirmation page or email), the watermark can include customer-specific data such as the customer’s first name, last name, and email. Your watermark is highly customizable and manipulable!
See all features, requirements, and limitations of the extension here.