This is an interesting discussion (off topic, but interesting).
In the many examples I gave for the sciences, the US government or the publisher, whomever holds the copyright, has agreed to let it be distributed without restrictions immediately of within a certain time after publication. Just because an article is copyrighted (they all are) doesn't mean that distribution is restricted. And at least for the conventional journals with which I am familiar, you (the author) cannot assert any rights that limit distribution, either. Of course in real life that is never an issue because the whole point of publishing your results (or reviews, or whatever) is to have them disseminated as widely as possible. But in the hypothetical case that you do, I do not believe you have that power. And in the US, if you have used grant money from the NIH I know you do not have that power. I suspect this applies to grants from other agencies of the government. I see the NSF has a similar policy (free access after 12 months is mandatory).
https://www.nsf.gov/pubs/2016/nsf16009/nsf16009.jsp#q1.
Note that I am speaking about academic journal articles in major disciplines including medicine and biology and the "hard" sciences like physics, astronomy, etc. I'm not talking about creative writing, where intellectual property is a concern. In my examples the goal is to get the information into as many hands as possible at no charge (costs underwritten by whomever funded the research).
And back to the context of this thread -- for many if not most Bookends users, the exchange of articles that publishers have made freely available (i.e., not behind paywalls) is entirely legal and in fact to be encouraged. This is obviously not the case for PDFs of articles or books for which the publisher asserts their copyright protections. Bookends uses are obviously highly skewed to professionals, and they understand what is and what is not the standard in their field (and can instruct the occasional Bookends-using grad student in their lab).
To summarize: a copyright does not imply that use of a PDF is restricted in any way. That is only the case if the publisher asserts such a protection. For Bookends users in many disciplines these protections are never asserted or are asserted only under certain conditions (e.g., no distribution by 3rd parties for the first 6-12 months). It's up to the user to understand what is and what's not allowed, which I suspect the vast majority of Bookends users already do. This thread would go differently for apps aimed at high school or college students, for example.
I suspect that I''m obligated to mention that I'm not a copyright lawyer, so don't rely on this post in a court of law!
Thanks for the give and take. If anyone wants to continue this I suggest that it be in a new thread.
Jon
Sonny Software