As far as I know, all you need to teach a PDC is the certificate from when you took your PDC. There are, of course, other certificates you can get such as a peer review of your work, but they are not necessary. This is what makes Permaculture so great, you don't have to jump through giant hoops(some even in flames!) to get out and teach. As long as you follow the curriculum as it was laid out by Bill Mollison in Permaculture: A Designers Manual, you'll be okay. You can always teach extra stuff though, such as highly detailed information on a specific environment, cob house construction, etc.
It probably would be a good idea if you've actually done some permaculture work and not just studied it, though.
I think
this explains it the best.