The PDF creator must enable usage rights for Reader users ( File > Save As Other > Reader Extended PDF).
Acrobat and Reader can embed the required elements, if the elements are available. If the required elements are available and embedded during signing, the signature can be validated requiring external resources for validation.
The required elements for establishing the validity of a signature include the signing certificate chain, certificate revocation status, and possibly a timestamp. Without conforming revocation status, the signature cannot be validated. Once a certificate expires, the issuing authority is no longer responsible for providing revocation status on that certificate. This limitation occurs because certificates related to the signature eventually expire or are revoked. Without certain information added to the PDF, a signature can be validated for only a limited time. Embedding these elements can occur when the document is signed, or after signature creation. To achieve long-term validation, all the required elements for signature validation must be embedded in the signed PDF. Long-term signature validation allows you to check the validity of a signature long after the document was signed.
Its no longer possible to download it from Adobes website and Adobe wont be releasing any new versions or security fixes for it.dditionally, as of January 12, 2021, even if you have.