Error when trying to download and open a PDF

I was getting the next error while downloading a PDF in Internet Explorer 6. The sy开发者_开发问答stem tried to open the downloaded file automatically but I was getting:

"There was an error opening this document. This cannot be found"

This happens when IE for some reason decides the content is not cacheable - it downloads the file, immediately deletes it, and then tries to open the file that was just deleted.

I had the same problem when mod_rewrite added a "Vary: Host" header. As you discovered, making sure the content is cacheable resolves the problem. In our case, I simply suppressed that Vary header.