code: public InputStream getInvoicePdf(final String invoiceId) {
          if (invoiceId == null || invoiceId.isEmpty())
              throw new RuntimeException("invoiceId cannot be empty!");
  
          return doGETPdf(Invoices.INVOICES_RESOURCE + "/" + invoiceId);
      }
docs: Fetch invoice pdf
  <p>
  Returns the invoice pdf as an inputStream
  
  @param invoiceId String Recurly Invoice ID
  @return the invoice pdf as an inputStream