Billit does not yet have a separate pro forma invoice feature but you can easily create pro forma invoices by changing the layout of a quote, delivery note or order form.
Create a new corporate style for a quote, delivery note or order form and change the title and name of the template to 'Pro forma invoice'.
When you create a new quote, delivery note or order form choose the layout 'Pro forma invoice' to convert the document to a pro forma invoice. You can still convert the document to a real invoice later by selecting it from the list and clicking the orange 'Invoice' button.