alert message is not displaying after downlading the file

Question

$select = "SELECT * FROM `success-stories` where id='$id'";
$result = $conn->query($select);
$row = $result->fetch_assoc();
$file=$row["download_pdf"];
$file_arr=explode("/",$file);
$path=$file_arr[0]."/".$file_arr[1]."/".$file_arr[2];
if(file_exists($path))
{
header('Content-Description: File Transfer');
header('Content-Type: application/pdf');
header('Content-Disposition: attachment; filename="'.$path.'"');
header('Expires: 0');
header('Cache-Control: must-revalidate');
header('Pragma: public');
header('Content-Length: ' . filesize($path));
ob_clean();
ob_flush();
readfile($path);
exit;
}
echo "<script>alert('Thank you for downloading story..');</script>";