You would need to consult with VeriSign to determine the actual product you should invest in, and how to implement their PKI.
That being said, a digital signature on a PDF is created using an x509 compliant digital certificate. These certificates can be self-signed (created with Acrobat Pro or some other third party utility) or by a Certificate Authority such as VeriSign.
You may also want to look at "electronic" signatures that are supported on PDFs using the signature field object and usually a plugin to Reader or Acrobat. Check out the "electronic and digital signature hardware and software" category at the link: http://www.adobe.com/security/partners/index.html
Regards
Steve