If the document is encrypted and specifically requires Adobe to open it, I’d first make sure you’re opening it in the Adobe Acrobat Reader desktop app, not the browser’s built-in PDF viewer.
Also, try downloading the file again and opening it directly from your Downloads folder. If Acrobat shows an error message when you try to open it, the exact message would be useful because encrypted PDFs can fail due to permissions, certificate/security settings, or an issue with the file itself.
If other PDFs open normally but this particular encrypted document does not, I’d also ask the sender to confirm that the file was encrypted for your Adobe account/certificate and that the document isn’t restricted to a specific recipient.