Hi William
Hope you are doing well and sorry for the trouble. As described the PDF Portfolio appears blank to some users on 20.13.20064 unless they go to view > protfolio > coversheet. and the same files open fine on your computer
The optional update with version 20.13.20066 in now available for Adobe Acrobat/Reader DC (Windows Only), addressing some of the issues reported with version 20.13.20064. Please ask the users to update the application to the new version from the help menu > check for updates or they can manually download the patch using the link: https://www.adobe.com/devnet-docs/acrobatetk/tools/ReleaseNotesDC/index.html and see if that helps.
Also, I have tried to reproduce te issue on my end with the latest version and the application is working fine.If the users still experience the same after updating the application please share the file with us for testing. Please upload the file to the document cloud https://documentcloud.adobe.com/link/home/ , generate the link and share it with us.
Regards
Amal