Hi,
The "additional information" area is only filled with custom fields of the table "order".
You can add the code:
<?php
JPluginHelper::importPlugin('hikashop');
JPluginHelper::importPlugin('hikashopshipping');
JPluginHelper::importPlugin('hikashoppayment');
$dispatcher = JDispatcher::getInstance();
$dispatcher->trigger('onAfterOrderProductsListingDisplay', array(&$order, 'order_front_show'));
?>
in the invoice.php file of the plugin and then other components like HikaSerial should be able to add their own data to the PDF invoices.