I'll leave this idea for now, and revert to hyperlinked images.
This is the right answer. You can do some pretty incredible things in PDFs, but anything beyond 'text and pictures on a page, mild clickage' is unfortunately unlikely to work for a wide audience.
Thank you for indulging someone who is on the edge of their expertise.
Windows 10 laptop/Firefox.
Your test pdf gave me a stern warning
'This pdf contains forms. The filling of form fields is not supported'
Salamander, my choice of pdf readers, seemingly opened the pdf, but gave no warning, but equally no hover-revealed item.
Thanks,
I'll leave this idea for now,
and,
revert to hyperlinked images.