Edit / Add Text or Stamp to PDF

Any suggestions how to accomplish the following:

When a PDF is moved from "Inbox" to "Completed" I would like to add a text/stamp including the date in the PDF itself. (Not to the name of the PDF). Eg. "Competed, 2016-12-01"

How could this be done?

You'll need an external script to do this. You'll also need to track down a program that can edit the contents of PDFs. My guess is that it may be quite difficult to do this consistently given the free form nature of PDF.