When you sign digitally, the document gets a digital timestamp. This is a legally valid timestamp from a TSA (Time Stamping Authority).
A signature stays legally valid forever. But it cannot be validated forever, because certificates expire after some time. This has nothing to do with the validity of the signature, only with the technical limits of the certificates.
Lawyers who want their PDF documents to stay verifiable for the long term can use LTV (long-term validation) and LTA (long-term archiving). These are rather expensive, specialised document management systems.