Hi,
Well, if you set the "show taxed price" setting of the HikaShop configuration to "no tax", the plugin will take it into account and display the price of the product as well as the subtotal without taxes on the PDF.
Now, you can copy the file of your layout from plugins/hikashop/attachinvoice/attachinvoice/ to media/com_hikashop/plugins/ via FTP and modify the layout to change only the subtotal.
There, you can change the line:
echo $currencyHelper->format($order->order_subtotal,$order->order_currency_id);
to:
echo $currencyHelper->format($order->order_subtotal_no_vat,$order->order_currency_id);