Use header() to convert to doc file, but some of them got empty page

[sunnyside]

I am using header() to convert to microsoft word file, most of files work fine; but there are some of file got opened by word with empty page or said "the file can't be found"

header("Cache-Control: no-cache, must-revalidate"); // HTTP/1.1
header("Expires: Mon, 26 Jul 1997 05:00:00 GMT"); // Date in the past
header('Content-Disposition: attachment; filename="'.$filename.'"');
header("Content-Description: $filename");
header('Content-Type: application/msword');

any tips?