Use this method to sign a signature field.
In order to sign a signature, you need to use your private key.
To authorize the use of this key, you need to provide your password
if you are not using a X509Certificate2 certificate.
Typically, this password protected private key is held in an
PFX/PKCS #12 (.pfx or .p12) file. So to perform the signing
operation, you provide a path to this file and a password to allow
use of the private key.
Time-stamped signatures can be produced by using the service of
a Time Stamping Authority (TSA). See TimestampServiceUrl.
If you are signing multiple signature fields in the same PDF
document, you should call Commit manually
after each signing operation.
If the file is not available, if the file is invalid or if the
password is incorrect, then this function will throw an