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.
Fourth, avoiding plagiarism is about properly apportioning intellectual credit, whereas copyright is about maintaining revenue streams.
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?
She asks her friends in other institutes if they have access to this article. That means that the institute they are working in have subscription to the journal in which this article was published. Among her friends person B has access to the article. Suppose she sends A an electronic copy of the article. A is happy, that she got the article. B is also happy, that he could be of help to A. But strictly speaking this is illegal. In the fine print all the publisher website have Terms and Conditions which we have to agree to (without reading them most of the times and they are written in legalese). These terms and conditions prevent us from sharing these articles from anyone else who might not have access to. For example for JSTOR the terms and conditions are listed here. If you read these finely what emerges is the way in which the publishers control the flow of information. For example it says:

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!