Is it OK to put a paper on the arXiv after it has been published online? Suppose that a paper has just got online in a journal without any previous submission to the arXiv. But the authors have spotted some minor typos and are also not satisfied with the typesetting of the online version. Can they put a newer version of the paper on the arXiv that contains some typesetting edits and minor corrections of typos?
This will depend on the terms and conditions that the journal published the paper under (and theoretically on the legal systems of the US and whichever country the journal publisher operates from). These conditions could range anywhere from "the publisher now owns the copyright, and you can't publish the paper anywhere in any form without their say so", to "put their typeset version up online anywhere you like". These terms should have been included or linked to when you signed for the final publication.

On average though, provided the publisher is willing for you to put out preprints, they typically want it to be your last version, not their typeset version, in which case there isn't likely to be any additional issue with the kind of differences you appear to be talking about.