How do I download help topics as a PDF for offline viewing, please?

This is indeed a regrettable decision.

However, you should be able to make your own help PDF. Select the URL  Acrobat Help | Acrobat User Guide

In Acrobat DC, choose File > Create > PDF from Web page.

You'll need to capture more than one level to get the whole Acrobat DC help PDF. I'm not sure how many levels need to be downloaded, to get all the Acrobat help topics, but maybe start with 5 or 6 levels. If that's too many, you can always delete extra pages later.