You might remember that we were still using PDF certificates at EDUlib until very recently. We moved from Koa Native to Nutmeg Tutor in mid-January this year when we were able to switch payment processor from Paysafe/Netbanx to Cybersource. That solved our Payment MFE issues and we took the plunge to HTML certificates by the same occasion. We still have the PDF certificates accessible for users who had those previously, but all our new certificates are HMTL based since January 21st, 2023.
Obviously, I remember Stanford was using PDF certificates too, but since they closed their instance about 2 years or 3 years ago, I don’t know who might still be using their fork of edx-certificates or if anybody is still using PDF certificates.
There is one thing I would like to point out though after reading some of the learner credentials stuff over the past few months: Honor Certificates are still being used.