Hi Steve,
Form what you have mentioned above, if I understand correctly the Acrobat Reader is not able to view the old pdfs or does it not showing the PDFs in list?
What is the exact version of Acrobat Reader that you have currently?
Adobe Acrobat and Reader official update containing the fix for this issue is now available. This update will be automatically pushed to all existing installations of Acrobat and Reader. If you want, you may also manually trigger the update by opening the application and going to Help > Check for Updates.
More information about this release is here: Acrobat Help | Release notes | Acrobat DC and Acrobat Reader DC | Update
Let us know if doesn't work.
Regards,
Akanchha