I know this, but I want save invoices as invoice[$invoice_id].pdf
Here is function for saving invoices:
The problem is not your remote function here. As ispconfig billing module does not support slashes in invoice numbers in the rev3 release, the pdf invoice can not be saved to disk when you call the finalize function of the remote api or click on finalize in the interface. The function billing_invoice_get_pdf(...) is just trying to read the pdf file from disk that was created before and then hands it over to your script. So when the pdf can not be saved to disk in the first step, then you can not retrieve it with the billing_invoice_get_pdf later.
It might be possible to replace the internal get_invoice_filename(...) function of the interface/lib/classes/billing_functions.inc.php with the one from the rev 4 release. If you want to try that, send me an email to the address were you received the billing module from and I will send you the rev4 version RC.