In Seller Admin to go Manage Products > Edit Products, then select the product and click View/Edit Product. Scroll down to the Integrations section and click the PDF Stamper button. If Enable Stamping isn't ticked there, tick that box (and this page explains the other optional settings there), then scroll down in that box and click Save. This is our latest Stamper integration method that can work with mass-email Updates.
Your product may have been set up with an older method of PDF Stamping, so scroll down further to the Advanced Settings section and see if Webhook Integration is ticked with a Product Notification URL like this:
If so, that's our older PDF Stamping setup, so once you enable the new method as explained above, you can just delete that URL and untick the Webhook Integration box, then click Submit to apply changes to the product settings.
If you wish, you can use Send free download link to send yourself a test link for the product to verify PDF Stamping is still working.