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

/**
 * Change PDF invoice information meta.
 *
 * @param array         $info    Invoice info meta.
 * @param BEWPI_Invoice $invoice Invoice object.
 *
 * @return array.
 */
function change_invoice_information_meta( $info, $invoice ) {
	$info['invoice_number']['title'] = __( 'Quote #:', 'woocommerce-pdf-invoices' );
	$info['invoice_date']['title']   = __( 'Quote Date:', 'woocommerce-pdf-invoices' );

	unset( $info['order_number'] );
	unset( $info['order_date'] );

	return $info;
}

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