Someone who has an expired PR card and/or is not in compliance with the RO no longer has a 'valid' landing paper - but neither does it ever expire, once landed. After landing, the (signed, dated by CBSA) COPR is a record of landing. An historical document.
It does help establish that the individual was a PR, and therefore probably still one (not that many actually lose PR status), and provides the info necessary to look the person up in the CBSA/IRCC databases. (There are some much older records of individuals that may not be in the databases, but by now fairly rare).
So if the individual has that doc, and the databases don't show that the individual lost PR status (renounced/revoked), all that remains is for the individual to provide reasonable identification (that they are the person in the COPR/landing record). That basically meets any reasonable standard for demonstrating that one is a PR - and shall be admitted.
Granted, they might ask lots of questions of someone not in the databases (esp if PR file very old, since likely means very out of compliance) and spend some time confirming identity.