I found a solution to the dropbox problem,
replace www with dl and dropbox withdropboxusercontent in your URL. For example:
https://www.dropbox.com/s/xttkmuvu7hh72vu/MyFile.pdf
Becomes:
https://dl.dropboxusercontent.com/s/xttkmuvu7hh72vu/MyFile.pdf
To force a file download, append a direct link with ?dl=1. Example:
https://dl.dropboxusercontent.com/s/xttkmuvu7hh72vu/MyFile.pdf?dl=1
From http://tricksbuzz.com/get-direct-link-dropbox/
