I agree with what Jason mentioned above. Read permission means, you have just read access to the "Source". You won't be able to do anything else in there. Preventing someone from downloading is a different topic.
BTW, you could create a new view so that users won't see the menu from the document library items to be able to get to the Download option. This is one way, but it doesn't necessarily mean you're preventing the user from downloading the document. If someone
wanted, it can still be downloaded or Save the Document in their desktop after it's opened for viewing purpose.
Pman
http://www.pmansLab.com/