I'm not quite sure what you need from me. I can't really "give you an overview", you either have the skills you need to write it, or you don't. I spent quite a while discussing this with you, and I've already pointed you at the code in the form email plugin which does this kind of stuff, that could probably be adapted and used in a cron plugin.
I also made it fairly clear I don't think you can do what you need from cron, that the best approach is the one I described, which would be an extra option added to the email plugin, to allow you to "stash" the PDF, and just pick it up and send it when the IPN 'success' callback happens. Or you could do the same thing out of cron. But trying to build fully formatted emails from scratch in cron (or in the IPN handling) would require quite a fair amount of coding.
-- hugh