in the following template file (application/views/quote_templates/pdf/InvoicePlane.php) add this where you would like it to display on the invoice
in your /application/views/invoice_templates/pdf/ then this should change to invoice and not quote ($custom_fields[‘invoice’][‘PO’])):
<?php if(!empty($custom_fields['quote']['PO'])): ?> <?php echo 'PO Number' . ':'; ?> <?php echo $custom_fields['quote']['PO']; ?> <?php endif; ?>