Self-signed digital signatures are precisely that - the person creating them is the only one attesting to the contents, so you can make a perfectly-valid self-signed ID for Canta Claus of you want to. The critical thing to remember is that a self-signed ID will only validate if the recipient has your keyfile to compare it to. On your own machine it will show as valid because the key is present, but if you send the PDF to anyone else it will show as invalid unless you have separately transferred them a copy of your keyfile. It's that second file which tells them the ID is really yours, as they can physically check where it came from (e.g. by phoning you up). The recipient would then have to manually add the keyfile to the trusted list in Acrobat or Adobe Reader, and finally your PDF signature will get the green tick.
Self-signed IDs are find for internal company workflows as everyone can share their keyfiles, and the IT department can manage what's going on. If you're using digital IDs in a public setting you should never use self-signed certificates, instead you should purchase an ID from a Certificate Authority - a company whose IDs are tied to the 'root certificates' embedded in Acrobat and Adobe Reader. The CA will require proof of identity before selling you the cert, and so anyone can verify it's genuine without needing to contact you. CAS-issued certs for signing PDF files are not cheap, there are several vendors out there and I won't comment on which may be better.
So basically anyone can open this pdf, create a signature file with my name and email and insert it into the pdf file then email it and the user the receives the pdf has no way of verifying that I was the actual person that signed it?
The important thing is that working with signatures is a multi step process
1. Send your public keyfile to the person who will be working with you.
2. They import it.
3. You each acknowledge that this has happened.
4. Send a signed file.
5. The recipient uses the keyfile they received earlier to check the authenticity.
In other words, there needs to be a "trused" communication at the start of the process where identity is actually verified. Perhaps a phone call or personal visit.
Thereafter the keyfile is used as a shorthand to replace double checking with the original human.
Trying to combine this in one step, sending the keyfile and PDF together, or sending the PDF alone where there is no keyfile to check against is - you are absolutely right - of no value at all.
Unless you use a certificate issued by a CA, which is the reason they cost so much.
Test Screen Name wrote:
The important thing is that working with signatures is a multi step process...
Note that there is no need to send a separate file (what has been called a keyfile in this discussion), as the recipient is able to extract it (user's certificate, aka public key) from a signed document, though it may be a good idea. As T.S. Name said, it is a matter of trust. If the recipient chooses to trust that the signed document they receive came from you, then they can feel free to extract the certificate from a signature and add it to their list of trusted certificates. The signature and any future signatures signed with the same digital ID can then be fully validated. I hope it's obvious that a user should not choose to add a certificate to their list of trusted certificates unless they trust the source of the document or other file that contains their certificate. There are many scenarios where such trust can be easily and reasonably established.