Hi,
By default, HikaShop has no PDF invoice. The only way to be able to generate PDF invoices directly in HikaShop is to use this PDF invoice plugin.
So since you said in your first message that you want to customize the PDF invoice, we deducted that you already had purchased that plugin and you wanted to know how to customize the PDF generated by that plugin, and that's why we answered like that.
If you're not already using that PDF invoice plugin, then we don't know how you're generating the PDF invoices and thus we can't tell you how to customize something we don't know how you're doing it. And that's why Jerome asked you to elaborate and provide more information on the situation.