Hi,
The PDF invoice uses a special "invoice.php" file that you can override by copying the file
plugins/hikashop/attachinvoice/attachinvoice/invoice.php
into
media/com_hikashop/pluginsinvoice.php
This new file will be used by the plugin you can edit it as you want.
Regards,
Jerome - Obsidev.com
HikaMarket & HikaSerial developer / HikaShop core dev team.
Also helping the HikaShop support team when having some time or couldn't sleep.
By the way, do not send me private message, use the "contact us" form instead.