An encrypted document with an Elliptical Curve Cryptography (ECC) certificate fails to open although the relevant certificate is installed. The following error message is displayed:
Beginning with the July 2016 release, Acrobat and Reader strictly conform to RFC 5753, which affects the Encryption with Certificates feature when Elliptical Curve Cryptography (ECC) is used to encrypt a document. The document encrypted using ECC certificates on July 2016 or later release of Acrobat or Reader fails to open in older versions of Acrobat and Reader with the above error message.
To open the document, update your Acrobat or Acrobat Reader to the latest version. For more information on how to update, see Install updates for Acrobat and Reader.