Cannot open old pdf files downloaded with ADE

I have ADE 4.5.10.186048 and I cannot open some old e-books (pdf's) downloaded on my older computers (no longer available) from various sites with older versions of ADE. The sites no longer exist and the message displayed by ADE is "Unable to add document Errors were encountered in this document". I can neither access the documents with Acrobat 2021.001.20150 as I receive the following message: "This document requires digital rights management (DRM) features supported by Adobe Digital Editions. Would you like to open it in Adobe Digital Editions now?". However, when I try to do that, I receive the ADE error message mentioned above.
Thanks in advance for your help.