session_start() messes up PDF download through PHP in IE8

I use this code to send an PDF through PHP to the visitor ([link][1]):

session_start();

$fileName = '0-1.pdf';
$file =开发者_如何学Python '../invoices/'. $fileName;
if (file_exists($file)) {
  $size = filesize($file);  
  header('Content-Disposition: attachment; filename="'. $fileName .'"');
  header('Content-Length: '.$size);  
  header('Content-type: application/octetstream');
  #header('Content-type: application/pdf');
  echo file_get_contents($file);
}

It works well on Chrome, FF and IE 9 but fails on IE 8.

The reason it fails is

session_start();

If I leave it out, it will work in IE 8 as well.

Unfortunately I need the session. Since the whole reason for passing the PDF through PHP is to perform an authentication check. So I need the session ID to check if the current user is allowed to access the PDF.

Any ideas how to get the download to work in IE 8 with the headers session_start() sends?

---

Just found the solution.

Aftter session_start() I have to set theese headers to get it to work with IE 8:

header("Cache-Control: must-revalidate, post-check=0, pre-check=0"); 
header("Pragma: public");

Strange behaviour... But it works now :-)