Hi Peter,
We have this problem also, our instructors like to use powerpoints and pdf files to share info that they will be teaching in their live events but don't want them to be downloaded.
The only way we could prevent this was to sign up to a 'flip-book' service which turns the pdf into an interactive page, essentially HTML5.
This gave us the option to 'prevent download' on the documents.
Obviously it depends on the files you need to restrict.
Hope this helps!
Tomos