A friend gave me a PDF document with Certificate encryption, he bought from a vendor. The PDF even does not allow copy text, so he wanted me to crack it. After effort, I removed the encryption. And I also developed a software to do this kind job without need Adobe Acrobat, but I am not sure if it is legal or not, so I can not release it until I know it is legal.
BTW, Digital Signature vs (Certificate) Encryption
A digital signature can guarantee the integrity of the message and the identity of its sender. Encryption can prevent someone from being able to read an intercepted message.
Update: just noticed that LiveCycle Workbench ES has the feature, Remove PDF Certificate Encryption
Returns an unsecured PDF document when given a PDF document encrypted with a certificate. After certificate encryption security is removed from a PDF document, it is no longer secure.
Remove PDF Certificate Encryption