Sharing files with Google Drive can be very convenient, but the link that google generates, something like this:

https://docs.google.com/open?id=0B2XdrMbOharQVnFyXDFGHYNYTTg

will just open the file for preview with the browser. To generate a link for download, use this link format instead:

https://docs.google.com/uc?export=download&id=0B2XdrMbOharQVnFyXDFGHYNYTTg

And change the part after id= to match the link that Google provides.

Similarly you can do the same thing for Dropbox, just add ?dl=1 to the end of the url that Dropbox provide.