It is in the interest of the publishers to confuse plagiarism with copyright. And many people wouldn’t know the difference. So here is a difference between the two:

First, plagiarism is a violation of academic norms but not illegal; copyright violation is illegal, but in truth pretty ubiquitous in academia. (Where did you get that PDF?)
Second, plagiarism is an offence against the author, while copyright violation is an offence against the copyright holder. In traditional academic publishing, they are usually not the same person, due to the ubiquity of copyright transfer agreements (CTAs).
Third, plagiarism applies when ideas are copied, whereas copyright violation occurs only when a specific fixed expression (e.g. sequence of words) is copied.
via Plagiarism is nothing to do with copyright

This would also relate to an earlier post, in making the difference between wrong and illegal. It can be exemplified in this case also.
Suppose for her research person A need a particular research article and she or her institution do not have access to it. What does A do?

Institutional Licensees shall make reasonable efforts to ensure that access to the Licensed
Content is limited to Authorized Users and to protect the Licensed Content from unpermitted use.

This clause essentially makes what happened between A and B illegal and just for sharing this article they might terminate the B’s institutional access to JSTOR. Now we can ask this question that whether the gesture on B’s part to help A was wrong and illegal both? As per definition by JSTOR this is clearly a violation of copyright. But what is the status of A’s research which emerges from this article given by B. Is it illegal? Can it be called as plagiarised (A gives proper citation of course)?

If you apply Kolhberg’s theory of moral development, the person who has the most developed morality will perhaps help the other without bothering about the copyright!

# Open Access Manifesto

Information is power. But like all power, there are those who want to keep it
for themselves. The world's entire scientific and cultural heritage, published
over centuries in books and journals, is increasingly being digitized and locked
up by a handful of private corporations. Want to read the papers featuring the
most famous results of the sciences? You'll need to send enormous amounts to
publishers like Reed Elsevier.
There are those struggling to change this. The Open Access Movement has fought
valiantly to ensure that scientists do not sign their copyrights away but
instead ensure their work is published on the Internet, under terms that allow
anyone to access it. But even under the best scenarios, their work will only
apply to things published in the future.  Everything up until now will have been
lost.
That is too high a price to pay. Forcing academics to pay money to read the work
of their colleagues? Scanning entire libraries but only allowing the folks at
universities in the First World, but not to children in the Global South? It's
outrageous and unacceptable.
"I agree," many say, "but what can we do? The companies hold the copyrights,
they make enormous amounts of money by charging for access, and it's perfectly
legal - there's nothing we can do to stop them." But there is something we can,
something that's already being done: we can fight back.
Those with access to these resources - students, librarians, scientists - you
have been given a privilege. You get to feed at this banquet of knowledge while
the rest of the world is locked out. But you need not - indeed, morally, you
cannot - keep this privilege for yourselves. You have a duty to share it with
requests for friends.
Meanwhile, those who have been locked out are not standing idly by. You have
been sneaking through holes and climbing over fences, liberating the information
locked up by the publishers and sharing them with your friends.
But all of this action goes on in the dark, hidden underground. It's called
stealing or piracy, as if sharing a wealth of knowledge were the moral
equivalent of plundering a ship and murdering its crew. But sharing isn't
immoral - it's a moral imperative. Only those blinded by greed would refuse to
let a friend make a copy.
Large corporations, of course, are blinded by greed. The laws under which they
operate require it - their shareholders would revolt at anything less. And the
politicians they have bought off back them, passing laws giving them the
exclusive power to decide who can make copies.
There is no justice in following unjust laws. It's time to come into the light
and, in the grand tradition of civil disobedience, declare our opposition to
this private theft of public culture.
We need to take information, wherever it is stored, make our copies and share
them with the world. We need to take stuff that's out of copyright and add it to
the archive. We need to buy secret databases and put them on the Web. We need to