Send content in mail + Rules When a user registers an account I would like to create an invoice (this is a content type) and send it as a pdf to the user. But I have no clue how I can send the invoice to the user... . 

I know you can send a mail, but how to include the invoice I have no idea.

I've searched on the internet but no luck ... .
I just had to implement this on a site in a similar fashion. You can use MIME Mail and it's submodule MIME Mail Attachments along with the Print and the Rules Download module. It will allow you to create a rule and email (with attachment.