We don't currently have a way of including attachments on email templates. In fact, that's been by design. Our email system is built around large-volume sending where we optimize deliverability (trying to avoid spam filters) so that all the transactional email sent out to guests gets read as much as possible.
That being said, we recommend that you reproduce the attachment as the body of the email. Is that possible in your case?
In other words, can you use our rich-text editor to make an email template with the pictures and words and fonts that look similar to your PDF? If so, then the attachment would not be necessary.