If you sign a PDF or other type of document with an expired document signing certificate, it will not be trusted by operating systems and software applications. You should renew your document signing certificate with your certificate authority (CA) before it expires.
Digital signatures can become untrusted by applications like Adobe Acrobat when the certificates used to create them expire or are revoked. However, you can use long-term validation (LTV) to ensure that all the information necessary to validate your signature at the time of signing is included in the signature. LTV allows your signature to remain valid long after the signing certificate has expired. For more information, please read Long-Term Validation (LTV) of PDF Digital Signatures in Adobe Acrobat.