Re-activating/re-sending expired links for a Stamped PDF product is also covered in the "A Few things to keep in mind" section of that help page:
4. Ordinarily, if you update a product's file and the buyer uses their original, unexpired download link again after that, they would get the latest file you'd uploaded for that product; however, if you enable PDF Stamping, each buyer's original download link will always issue their original Stamped file, even if you re-activate their link;
I.e., if you want to issue links to download a new edition of your file, you must issue each buyer a new link, rather than re-activating their old one.
To view your Buyer Group list for each of your products, log into Seller Admin and then visit this link directly while you're still logged in:
You may want to bookmark that URL, as it's linked in our help page for Updates and Newsletters but not linked in Seller Admin quite yet.