Send PDF Invoice sends default HTML invoice converted to PDF format to customer. You can set when the invoice will be set - right after order confirmation (invoice will be attached to order confirmation mail), when the order status is changed to the one that means order is complete (invoice will be attached to the notification mail - checkbox to notify customer has to be checked) or only after clicking button in order administration. Invoice can be sent whenever by clicking button in order view in administration.

More Info:
http://www.opencartex.com/opencart-modifications/send-pdf-invoice

NEW:
- customer can download created invoice from his account (can be enabled/disabled in store settings)