Is it ethical/legal to obtain journal articles from reddit?

Legally, I am not sure --- it depends on the laws on the country as well. Ethically, I don't see anything wrong about it. Academia is about building and sharing knowledge: if anything, I would consider it unethical to put papers behind a paywall.

Most scientific papers have their copyright owned by an academic publisher (which may be a for-profit company, a scientific society, a university press, etc.), and the publisher often does not allow people to redistribute the papers that they own. Hence, in most cases, it may be a violation of copyright for an individual to give you a copy of a scientific paper, either privately or by posting it online. Sometimes, even the authors of the work may not be legally allowed to share their work with you, because they have signed the copyright away -- even though this is common practice.

Now, in terms of ethics, many papers are produced by researchers who are paid by public money, and many researchers intend their work to be as widely disseminated as possible. In any case, researchers are not paid when publishers sell their articles, and few would object to their papers being shared with people who cannot afford to buy it from the publisher. People in the open access movement will argue that, in ethical terms, people like you should have access to the output of scientific research, without the publisher's copyright standing in the way. In fact, open access advocates often feel that academic publishers, in particular commercial ones, are acting against the interest of the research community by restricting the dissemination of scientific papers. This follows the intuition that research supported by public funds should be freely available to members of the public.

Personally, I would encourage you to obtain articles however you can when you cannot obtain them from publishers, without worrying about legal issues. I find that the most convenient way is often to use Sci-hub, but you can also ask for papers on Reddit, or asking colleagues or the authors of the paper. I can completely sympathize with the fact that it is essentially infeasible, when checking related work, to send emails, requests, wait, etc., for every single paper which you think may be relevant and need to check out. Situations like yours is what make me believe that the current academic publishing system has significant problems, and that encouraging open access to scientific papers is worthwhile.