Hello Elspethp,
We apologize for the inconvenience caused, as per the description above, you are not able to open PDF files using Adobe Reader on your Mac machine, Is that correct?
Try opening few PDF files with Adobe Reader by navigating to Window>Cascase>Tile and see if this brings up any PDF files.
You may also refer to the Adobe article and try the suggestions Can't open PDF
If you still experience any issue, please use Acrobat Cleaner tool to remove Adobe Reader Download Adobe Reader and Acrobat Cleaner Tool - Adobe Labs
Reboot the machine and install the latest version of Adobe Reader from Adobe - Adobe Acrobat Reader DC Distribution
Are you not able to open all the PDF files or is it specific to one PDF? If it is specific to one PDF file, Is it possible to share the PDF file with us so that we can test it at our end? To share the file, please use Adobe Send feature, upload the file, share the link to files via private message only, How Do I Send Private Message
Let us know how it goes and share your findings.
Regards,
Anand Sri.