How can I sign a PDF with an external signing service?
Signing PDFs with an AATL approved certificate, as required to get the nice green checkmark on signatures in PDF files for validity, typically requires signing through PCKS#11 and with a USB token, smart card or a HSM device.
Operating such devices may not be feasible for everyone, and therefore some parties choose to use a 3rd party service to create the signature. This is why UniPDF supports signing your PDF documents with the help of external devices.
You can check out the example here to learn how to use such a 3rd party service to work with UniPDF to send the hash to the 3rd party server, and then applying the signature to the final PDF.