You probably know about it already, but you'll probably want to use Unpaywall: https://unpaywall.org/ https://unpaywall.org/ It catalogues Open Access links gi

You probably know about it already, but you'll probably want to use Unpaywall: https://unpaywall.org/ https://unpaywall.org/ It catalogues Open Access links given a DOI, provides an API, and also provides links to direct PDFs (or web pages containing the complete article, if available).