How does Acrobat Reader react when the signer's certificate is expired?

In the tested versions, Acrobat Reader indicates the certificate can not be verified anymore for the validity period is over.

Ideally, Reader would be able to check the validity of the certificate at the signature date. But once the certificate's expired it does not appear in the revocation list anymore. Reader cannot tell if it is valid or not and so considers it as invalid and uncheckable.

You should then use a long lifetime certificate to sign documents that are made to last.