Signs the document using an external signing facility.
Use this method to create a PDF signature using an external signing facility for the cryptographic computations. SignExternal delegates the low-level signing operation to an external, remote, or custom signing engine. This method is useful if the signature has to be made by a device accessible through a custom or non-standard signing interface.
When all preparations are done and the hash is computed, the component fires ExternalSign event which allows to pass the hash value for signing.