Hi,
That's already possible by default without any plugin.
You can for example create a menu item of the type "hikashop user control panel" via the Joomla menu manager so that logged in users can access their orders. And if you have the HikaShop configuration setting "Allow users to print invoices on the front end" activated, they'll get a "print invoice" button on their orders.
Now if you have this PDF invoice plugin, then instead of an HTML invoice, the customers will get the PDF invoice as they would get by email.