A purchase order number is usually issued by the client when they agree a sale. In fact when they agree the quotation in their control panel they can supply a purchase reference along with any additional instructions.
IF this box is available in estimates and you have the tick box in the invoice layouts to show it it should show up in estimates as well as invoices. Otherwise you should give a different layout page for estimates and invoices so i can adjust each separately. Why you have one invoice layout for both I don’t understand especially if some of those features are not available in both layouts