There are 2 methods to delete proforma invoice, See below
NOTE : This operation can not be undo

Method 1:

Step 1 : Click on Other Documents Menu & select proforma invoice option from sub menu

Step 2 : Click on the down arrow beside the edit button then click on delete proforma invoice option

Step 3 : Click on Delete Button

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Method 2:

Step 1 : Go to Other Document Menu then select the proforma invoice sub-menu

Step 2 : Select the checkbox before invoice number & click on the “Delete” option at bottom of page to delete selected invoices.

Step 3 : Click on the “Delete” button to verify delete operation.