tcpdf and digital signature certification

I'm trying to do a simple pdf certificate signing in php with tcpdf.

I'm running example 52 but the information on my pdf reader (osx preview) says:

开发者_StackOverflowEncryption: No

Permissions: You have full permissions for this PDF. You can copy text, print or make changes to the PDF and save.

And clicking the signature logo does nothing.

Am I doing something wrong or is this just plain broken?

Ok, apparently it works.

The problem was that OSX's preview doesn't show certificates.

I opened the PDF with Acrobat Reader and there it was.