The WP PDF Stamper plugin allows you to dynamically stamp a PDF file with the customer’s details (e.g. name, email, address etc.) upon purchase. The stamper then sends the stamped copy of the PDF file to the customer via email. If you are wondering why it is important to stamp the PDF eBook before giving it to your customer then this page is for you.
Additionally, you can password protect the stamped PDF files with the customer’s email address and encrypt the PDF files to prevent users from being able to copy, modify or print the content of the file.
More details here.