Does Aspose.PDF support the use of an external signature server?
In such a case the Message Digest (the checksum of the PDF-content) is built on the client. Then the built Message Digest ist sent to a server as byte-array, where the Message-Digest is signed (eg. using a HSM). The Signed Message Digest is sent back from the server to the client where it is embedded to the PDF.
Details are described in the iText-ebook digitalsignatures20130304.pdf which can be found here:
Please take a look at chapter 4.3 and especially 4.3.2.