Fails:
HTTP/1.1 200 OK
Date: Wed, 02 Jul 2003 12:49:07 GMT
Server: Apache/1.3.27 (Unix) Debian GNU/Linux PHP/4.1.2 mod_perl/1.27
Content-Location: how.pdf
Vary: negotiate,accept
TCN: choice
Last-Modified: Tue, 01 Jul 2003 19:36:44 GMT
ETag: "469efd-2428-3f01e2cc;46ac64-d9-3f01d981"
Accept-Ranges: bytes
Content-Length: 9256
Keep-Alive: timeout=15, max=100
Connection: Keep-Alive
Content-Type: application/pdf
Expires: Wed, 02 Jul 2003 12:49:07 GMT

Works:
HTTP/1.1 200 OK
Date: Wed, 02 Jul 2003 13:00:53 GMT
Server: Web
Last-Modified: Wed, 17 Oct 2001 22:09:02 GMT
ETag: "5d813-11c34-3bce017e"
Accept-Ranges: bytes
Content-Length: 72756
Keep-Alive: timeout=15, max=100
Connection: Keep-Alive
Content-Type: application/pdf