The problem was that the shareable links feature has to be activated first for your account by following
https://www.dropbox.com/enable_shmodel .
The problem with
https://dl.dropbox.com/s/5t9r3l91bju...tions.pdf?dl=1 is, that it doesn't have .pdf extension and thus doesn't qualify for downloading. I think I can make a workaround for that.