Download not working for path

I have the following situation that needs your support.

When a path is created for an uploaded file, the link will not force the browser to download the file.
Here's an example:
http://www.deutsche-gesellschaft.de/fokus/stellungnahme-des-dgb-zu-syner...

The alias points to an PDF file, which is then displayed in the browser (gibberish) instead of being downloaded, like when using the direct http path to the file, which is:
http://www.deutsche-gesellschaft.de/sites/default/files/206-stellungnahm...

To make it even stranger, as long as I'm logged in, the upper link works fine, and forces the file to download. But not after having logged out.

Thanks for any input!