Staff users cannot send Proforma Invoice emails in bulk from Lightning Experience, but this is possible from Salesforce Classic. Revert to Salesforce Classic in order to send Proforma Invoices in bulk.
Navigate to the Sales Order tab.
Click Go! next to the View: All dropdown. The following buttons will display: New Sales Order, Rapid Order Entry, Print Proforma, and Send Proforma Invoice Email.
Click the checkbox next to the Sales Orders for which you wish to send emails. The system will only send emails for the Sales Orders that have the Posting Entity set to Receipt, Status as Unpaid, and Posting Status as Pending. Additionally, the Contact on the Sales Order must have the Preferred Email field populated.
Click the Send Proforma Invoice Email button.
Once the Proforma Invoice Email has been sent, the system tracks this status via two fields: Proforma Invoice Email Sent and Proforma Invoice Email Sent Date. The Proforma Invoice Email Sent Date will always display the date of the most recently sent email. If the email is sent again, the Sent Date will be updated.