Checks for free PDFs from Google Scholar, Sci-Hub, LibGen, Anna's Archive, Sci-net, Semantic Scholar, Unpaywall, OpenAlex, Openrxiv (medRxiv/bioRxiv), and ArXiv. When hovering a DOI, it also displays journal name, ISSN, publisher, metrics (SJR, H-Index, JIF, CiteScore), citation count, and integrity status (PubPeer, Retraction Database, Beall's Predatory List).
