Unpaywall:

• “How do you find all these fulltext articles? We harvest content directly from over 50,000 journals and open-access repositories from all over the world. We also use great open data from PubMed Central, the DOAJ, Crossref (particulary their license info), and DataCite.
• Is Unpaywall legal? Yes! We harvest content from legal sources including repositories run by universities, governments, and scholarly societies, as well as open content hosted by publishers themselves. We do not harvest from sources of dubious legality like ResearchGate or Sci-Hub. If you ever encounter content indexed by Unpaywall that is posted in violation of copyright, let us know and we’ll remove it immediately.
• How can I make sure content from my open repository appears in Unpaywall? Good question! We made a page about that here.
• How is this related to the oaDOI service? We used to call the browser extension “Unpaywall,” and the data source behind it “oaDOI.” That got confusing, so now the whole project is just called Unpaywall, including the database, the API, the extension, and everything else…”