this is the function where pdf names are created.
it's in apps/siwapp/modules/print/actions/actions.class.php, on line 102
  /**
   * PDF output
   * @param array ids - Object IDs as request parameter
   */
  public function executePdf(sfWebRequest $request)
  {
    $model = $request->getParameter('model');
    switch($n = count($ids = (array) $this->getRequestParameter('ids', array())))
    {
      case 0:
        $this->forward404();
        break;
      case 1:
        $name = $model."-{$ids[0]}";
        break;
      default:
        $name = "$n-".$model."s";
        break;
    }
    try
    {
      $this->render($model, true)->stream("$name.pdf");
      return sfView::NONE;
    }
    catch(LogicException $le)
    {
      throw $le;
      //return $this->renderText($this->templateNotFoundMsg);
    }
    catch(TemplateNotFoundException $tnfe)
    {
      return $this->renderText($this->templateNotFoundMsg);
    }
  }
you can change the value of the "$name" variable just before the call to $this->render($model, true)->stream("$name.pdf");
-- 
uh, 
oh.