I checked the mime type of pdfs meanwhile and found the 
following using the debug mode:

response for 
www.asjkldfsjdfjksd.com/docroot/_dynamicscript/pdfdownload.p
hp?file=%2Finvestorhandout1032339900%
2Firhandout_q3_2002_web1037202626.pdf&name=IR-
Handout_Q3_2002_web.pdf:
code=200
>>> HTTP/1.1 200 OK
>>> Date: Mon, 18 Nov 2002 16:52:09 GMT
>>> Server: Apache/1.3.26
>>> X-Powered-By: PHP/4.2.2
>>> Content-Disposition: attachment; filename="IR-
Handout_Q3_2002_web.pdf"
>>> Expires: 0
>>> Cache-Control: must-revalidate, post-check=0, pre-
check=0
>>> Pragma: public
>>> Connection: close
>>> Transfer-Encoding: chunked
>>> Content-Type: application/octet-stream

may the mime type be the reason for the problems? i also 
checked if some url contains a time stamp or a variable, 
but it does not seem to.