In order to add invoice information meta to the PDF invoice template, simply use below filter function as an example and add it to your themes functions.php file.

/**
 * Add PDF invoice information meta.
 *
 * @param array         $info    Invoice info meta.
 * @param BEWPI_Invoice $invoice Invoice object.
 *
 * @return array.
 */
function add_invoice_information_meta( $info, $invoice ) {
	$info['po_number']['title'] = __( 'PO Number #: ', 'woocommerce-pdf-invoices' );
	$info['po_number']['value'] = get_post_meta( $invoice->order->get_id(), 'YOUR_META_KEY_HERE', true );

	return $info;
}

add_filter( 'wpi_invoice_information_meta', 'add_invoice_information_meta', 10, 2 );