I got all excited - then realised "signing" just means inserting a picture. Notably absent are open source tools for digitally signing and verifying PDF's. Apparently pdftk does it in a paid version.

It's funny in a way - in this thread we have people wanting ways to modify a PDF. Yet to me, being any to prove it's not modified (eg, it's statement provably issued by some bank saying they transferred funds to my bank on behalf of person XYZ) is far more important. Instead we have companies offering paid "document signing services" which are built on sand - you can easily forge / modify any signed document they issue.