Server-based PDF Signing using HSM

I want to implement a solution for server side pdf signing using network-attached hardware security module. my first question is : what is PDF Signing Certificate? Certificates are public keys, how a certificate can be used for signing? from http://ift.tt/1y38fiB:

With the server-based PDF Signing, Certifying and Approving PDFs require a 
GlobalSign-issued PDF Signing Certificate, stored and protected 
on a Hardware Security Module (HSM), and an automated 
digital signature solution to apply the
signatures to the documents.

Each user should have it's own private key. i don't want to use tokens for each user. can i generate and store user's private keys on the HSM? I doubt...is non-repudiation guaranteed with this solution?