This is an interesting introduction to digital signatures under Acrobat, however make sure you also open the PDF file and read the notes there.
PDF’s self-signature principle uses x.509 certificates, which rely on a certification authority (you in the case of self-emitted certificates) for identity verification purposes. This means only one person or company is responsible for issuing your digital identity. Of course you can decide you trust this one entity, but you also have to make sure others that will rely on your signature also trust them. This is easier to accomplish in corporate environments where such trust relationships can be enforced.
Another way to sign Adobe Acrobat documents (or *any* documents or file) is to use OpenPGP and verify the signer’s web of trust. An OpenPGP certificate (or [i]public key[/i]) can be signed by as many people/companies as you wish. Here is my web of trust, as listed in my Digital Signatures with PDF [PDFforLawyers.com] * Adobe Acrobat documents security overview [adobe.com] * Making and verifying signatures [gnupg.org]