Woocommerce Email dynamic attachments

Hi people,
I would ask you for your help. I want to send pdf invoices, which are generated with my invoice creating software and saved and uploaded as a pdf.
So the idea is to create the invoice as a pdf, save it and upload it with a specific name for instance invoice_”ordernumber”.pdf and send it with the “order completed” email.
I have the following code:

//Filter-Hook für Woocommerce
add_filter( 'woocommerce_email_attachments', 'widerrufsbelehrung_email', 10, 3);

function widerrufsbelehrung_email ( $anhang, $status , $bestellung ) {

$anhang_bei_status = array( 'customer_completed_order' );

  if( isset( $status ) && in_array ( $status, $anhang_bei_status  ) ) {

     $widerrufsbelehrung = get_template_directory() . '/PDF/Widerrufsbelehrung.pdf';

     $anhang[] = $widerrufsbelehrung;
  }

  return $anhang;
}

My question: how could I change the static attachment with a dynamic one?
How can I apply get_order_number()

Many thanks in advance
Bests