Hi,
I would recommend first to check the settings page of your custom field via the menu Display>Custom fields. In it, you'll find several display settings including one for the display on the invoice. So you need to make sure that this setting is enabled or the field won't appear on the invoice.
Also, If you're using an old version (from 5 years ago or more) of the PDF invoice plugin, it's possible that it doesn't contain the additional code we added to it to support these display settings.
In the file plugins/hikashop/attachinvoice/attachinvoice/invoice.php, you have the code:
<?php if(hikashop_level(2) && !empty($fields['order'])){
ob_start();
foreach($fields['order'] as $fieldName => $oneExtraField) {
if(!@$oneExtraField->field_frontcomp || empty($order->$fieldName)) continue;
?>
<tr class="hikashop_order_custom_field_<?php echo $fieldName;?>_line">
<td class="key">
<?php echo $fieldsClass->getFieldName($oneExtraField);?>
</td>
<td>
<?php echo $fieldsClass->show($oneExtraField,$order->$fieldName,'user_email'); ?>
</td>
</tr>
<?php
}
$additionalInfo = ob_get_clean();
if(!empty($additionalInfo)){ ?>
<fieldset class="hikashop_order_custom_fields_fieldset">
<legend><?php echo JText::_('ADDITIONAL_INFORMATION'); ?></legend>
<table class="hikashop_order_custom_fields_table adminlist" cellpadding="1" width="100%">
<?php echo $additionalInfo; ?>
</table>
</fieldset>
<br/>
<?php
}
} ?>
which handle the display of the custom order fields. So please check that you have this code (near the end of the file).