Proforma invoices become available for all orders that are not yet completed. They do not depend on the payment method i.e. they are available for orders that are about to be paid with PayPal or any other payment method.
Regular invoices are generated as soon as the order is marked completed or is auto-completed for the first time.
Both type of invoices are attached to standard WooCommerce emails. Proforma invoices are attached to “Processing Order” emails and regular invoices are attached to “Completed Order” emails.