Cannot open PDF completely

I work for an association that provides materials to our 16,000+ members (most are artists, not savvy computer users), some of the materials are PDF files.

I recently received calls from a flurry of members who are Apple users who indicated they suddenly cannot open PDFs (they were able to open them before). The members think our PDF files are corrupted, but I can open the PDFs (some old, some new) without issue using my PC with Chrome, Firefox, Edge. Also, I can successfully open the PDFs on my iPhone.

I sense that most of our members who Apple users and have complained are using Safari and a default PDF reader.

I have encouraged them to use Chrome and/or Adobe Reader, which works for some. I've also learned that in Safari, they can select "Preferences" and remove "Adobe Reader" from both "Security Plugin Settings" and Extensions" but I have no idea where to instruct them to find "Preferences" (our users need a lot of hand holding). UGH!

We have no IT staff and no Apple computer. I want to help these people resolve their issue, but don't know what else to suggest. Ideas anyone?

Thank you for your suggestions!

Kim