I spotted a 2019 paper which literally copy-pastes paragraphs from a 2011 paper and doesn't cite them. The author (who is different than the 2011 paper's) claims that as their own research paper. Should I report this to the journal?
The paper has six pages of copied content. Out of 46, 6 pages are complete copy-paste.
Yes. Such a grave case of academic misconduct should have publicly visible consequences. Let the editor know; the journal should issue a retraction as the whole article can be deemed to be unreliable.
As retractions can take a long time, it would also be useful if you comment on the suspicion of plagiarism on PubPeer. (The authors could respond with a defense there if needed.)
Yes, draw it to relevant attention.
Not least because if its a mistake the authors should have a chance to fix it; if (more likely) it isn't then who knows what the implications will be, down the line, of dishonest papers. People could spend years of their life doing work, only to find its invalidated and wasted, because underlying material was unreliable. Or, you don't say what the field is, but real harm could arise in many cases. People getting hurt or at risk, due to faked or inappropriately copied material (do you know what a plagiatlrist - a dishonest academic writer -will do next?!). Businesses and products based on these in part,not knowing the authors are dishonest.
You may want to consider best ways to disclose - who to tell and how to best approach it. This falls under academic whistleblowing.
But that's a different question.
Should you make the publisher, or some other relevant academic/s aware, in some suitable way - absolutely. Even if at worst, its simply an anonymous post showing the 2 papers side by side so it's unmistakable, and a link sent to various places where it'll do good.
(I don't necessarily know if that's the right way, but I use it to show that even if fearful of repercussions, there's a way. You need to ask how to disclose, too, but that as I said is a different question.....)
As others suggest, I would report this, but - depending on the circumstances, and if you want to help the author save face somewhat, you might contact them first. You wouldn't ask "Have you plagiarized this section from that paper?" But rather "I've been reading your paper X and believe that sections Y might be a verbatim quote from paper Z. Is this correct?" ... then depending on their answer you'll know how to report this to the publisher.
In cases of academic misconduct where you are not personally involved, it's often better to go do something useful instead of filing complaints.
There's lots of plagiarized papers out there that nobody's going to read. Limit your complaints to academic misconduct that involves you personally or has importance to research or safety.
I would also try to check if the 2 papers are both graduate student papers where the 2 authors were graduate students at the same time for the same advisor. There are some subjects where the first 6 pages is boilerplate as things are getting set up. If the advisor tells the 2nd student to copy the first 6 pages from the 1st student's paper, that is lazy but not exactly dishonest.
