> If you do this in a 
> browser, everything works well and you see for example 
the 
> PDF in the browserplugin 

First: wipe all "MIME types" default options, the default 
is to consider asp as html.

If it still does not work, check that the correct MIME type 
is given during responses.
A good test: is it working with mozilla or netscape?If the anwer is not, there
is no way you can make it work: 
IE is doing absolutely awful things to override the MIME 
type, trying to detect the binary flow type (and this is 
bad: see various .exe viruses tagged as text/html) but 
other standard (RFC) browsers don't.

If this still does not work, you can try to activate 
the "use old http/1.0 requests" option ; this will attempt 
to force the use of GET requests instead of HEAD ones, 
badly handled by some cgis.