Do you already have many accounts?
Maybe it would be easier for you to change the default currency?
https://help.whmcs.com/m/payments/l/1122770-changing-the-default-currency
The PDF file is generated dynamically, just like the HTML invoice, the client can view or download it many times.
You'd better use a hook when creating an invoice.
https://developers.whmcs.com/hooks-reference/invoices-and-quotes/#invoicecreated
If you have already created an Invoices for this service, a new Invoice will not be created.
You can change "Next Due Date" +1 day and try to create Invoice.