Hi!
in our journal (http://naerjorunal.ua.es) we integrated this option about two years ago. Unfortunately we didn’t develop it as an extensible plugin to other versions of OJS because it only covered our needs.
Certificate creation requires authentication (to avoid its use in open sessions). The PDFs are digitally signed. Attached are screenshots
A more modern alternative would be the inclusion of revisions in Publons’ author profiles. You can get this plugin for OJS 2.X. (https://github.com/publons/ojs)
In this post we also talk about other integrations of our journal: https://forum.pkp.sfu.ca/t/how-to-customize-ojs3-frontend-and-how-to-suggest-dev/28635/22
Regards