This is a new question of no relevance to the copyright issues that were raised earlier. It muddies the waters by conflating three separate copyright issues:

1. There are legal constraints on what can be made available on math.SE itself (that is, as downloadable material hosted on stackexchange.com, assuming it is a site in the USA).

2. There might or might not be significant legal constraints on what can be linked in math.SE discussions (externally hosted downloads). This would depend on many factors including differences in laws between countries.

3. The actual math.SE matter that prompted this question was been about whether offers to provide copyrighted material privately, through channels not directly involving math.SE, are a problem either legally or under the SO or MO policies. The answer appears to be that it's a total non-issue. Quoting from the other discussion:

'... one crucial difference is that posting a link places math.SE in the "action path" of a legal violation (the downloader normally clicks on it through math.SE) while posting an offer to engage in such a violation if contacted by other channels, does not. Until and unless there is legal advice that the latter activity jeopardizes SE's operations, or math.SE becomes a major piracy enabler on the radar of DMCA lawyers, shutting down such postings is gratuitous volunteer policing: self-censorship.'

It would be good for those who think that such private offers do raise legal issues, to explicitly address #3 in its own right as a separate question from #1 and #2.

This is a new question of no relevance to the copyright issues that were raised earlier. It muddies the waters by conflating three separate copyright issues:

1. There are legal constraints on what can be made available on math.SE itself (that is, as downloadable material hosted on stackexchange.com, assuming it is a site in the USA).

2. There might or might not be significant legal constraints on what can be linked in math.SE discussions (externally hosted downloads). This would depend on many factors including differences in laws between countries.

3. The actual math.SE matter that prompted this question was about whether offers to provide copyrighted material privately, through channels not directly involving math.SE, are a problem either legally or under the SO or MO policies. The answer appears to be that it's a total non-issue. Quoting from the other discussion:

'... one crucial difference is that posting a link places math.SE in the "action path" of a legal violation (the downloader normally clicks on it through math.SE) while posting an offer to engage in such a violation if contacted by other channels, does not. Until and unless there is legal advice that the latter activity jeopardizes SE's operations, or math.SE becomes a major piracy enabler on the radar of DMCA lawyers, shutting down such postings is gratuitous volunteer policing: self-censorship.'

It would be good for those who think that such private offers do raise legal issues, to explicitly address #3 in its own right as a separate question from #1 and #2.