Digital Signatures for PDF documents

Question

We are taking the user cert from the user's CAC (Smart Card) to grab their digital signature by using iText, which in iText code examples they use Keystore to grab the user certificate. Then place the user's digital signature onto a dynamic PDF to a specified field, which we've named it "USER_SIG". Issue we have run into is that it is not possible for us to grab the private key as it does not exist in the certificate we pull.

We know it is possible with Adobe Acrobat Reader DC, but we want to know if it is possible outside of Adobe without manually downloading the file to convert it to valid format of key store and sign the PDF document without a private key?

Answer 1

No, you cannot sign if you don't have any private key.
No, you cannot get the private key from the certificate (in fact, this is the reason for living of public key cryptography).
No, Adobe cannot sign without a private key.

If you have the private key inside an smartcard, you should use a pkcs11 keystore.