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.
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