Automated email to a new subscriber - Welcome etc

Hi, I want to send an email to a new subscriber saying Welcome... immediately after they have subscribed. The email would have an attachment - a pdf which is why they are subscribing, to get the pdf - and it is the only email that I would send through opencart. All other emails will be using MailChimp. So I don't need all that kind of functionality - I don't need email template design or tracking opens or interactions or anything like that. I just want to send a simple welcome email. Does anyone know an extension that will do that?