Copyright laws are the deterrent to somebody distributing downloaded PDF files and especially to them posting the files for free download. An infringer has no upside and a tremendous downside.
The WP and NYT haven't been successful at collecting money from their net subscribers as far as I know. No wonder, because the substance of their content is freely available in thousands of different ways.
Something like S.W.A.T. would have a better chance because its content isn't otherwise available. You can get it in a way the publisher endorses or you can do without it, and that's the exact key to the success of the print media over the years!
Just because a "successful" metaphor for e-distriibution doesn't exist or isn't well known yet doesn't mean it won't be figured out soon. The technology and equipment is in place for both vendors and consumers, so it's only a matter of time.