In addition to the note above regarding SSL/IE bug/Sessions, also make sure you DO NOT SET THE HEADER 'Pragma: no-cache' if you are sending an inline document (e.g., PDF document).

For example:

<?php

        header("Content-Type: application/pdf");
        header("Content-Disposition: inline; filename=foo.pdf");
        header("Accept-Ranges: bytes");
        header("Content-Length: $len");
        header("Expires: 0");
        header("Cache-Control: private");
//        header("Pragma: no-cache");//don't send this header!!

?>

Best,

--
Jim