From the Knowledgebase

Unexpected server response (204)

If you see this error message, do you have a download manager installed on your browser? This sometimes causes problems - maybe take a look at your extensions, and also try on a different browser to see if that works. If it does, it seems likely to be limited to your original browser.

Disabling the download manager should help, but if not please send us a link to the page containing the PDF.

You may also be able to configure the download manager not to intercept PDF downloads.