I tried this solution, but because my PDF files have customizing features that are enabled by Adobe’s digital signature, if you add new content to the document the signature will get invalid and the enabled reader features get lost. Acrobat will not let you edit a write enabled PDF files.
Another option is the SetaPDF-FormFiller API Lite (http://www.setasign.de/products/pdf-php-solutions/setapdf-formfiller/evaluate/)… all you need to do is just create a form field which is locked/read-only and fill it with the API. The only downside to this option is that this “stamp” is editable/removable by a customer who has Acrobat Pro.
If your DPF files are simple ebooks without any enabled features like forms that a customer can edit and save for example then SetaPDF-Stamper API is a great solution to protect your files.
Just a side note, my experience with their customer service and tech support was great.
You can contact my Magento developer if you want to try this solution, I think they are the only company who actually implemented this.