An iX article (8/2004, p. 116) mentions some quite strange Internet Explorer behaviour which affects dynamically generated PDFs – there’s an MS knowledge base article:
„To work around this limitation and to considerably improve the performance of your application, watch for the „contype“ userAgent request and only return the content-type.“
On an unrelated note, the same iX article points out that JSP (Java Server Pages) are – by design – unable to stream out binary data. Now how dumb is that? (Remember this for the next JSP vs. PHP debate…)