Use print_pdf in the background on node save?
The function used from that module to output PDF is print_pdf_generate_html(), which gets a filename as last parameter.

To understand what to pass as first argument, see print_pdf_generate_path(), which is the function calling print_pdf_generate_html(). In particular, see the following code.

$print = print_controller($path, $cid, PRINT_PDF_FORMAT);
if ($print === FALSE) {
  return;
}