For Open Access articles, Unpaywall [1] (or at least its API) usually can find a direct link to the PDF (or HTML if available). For example, this is what I use

For Open Access articles, Unpaywall [1] (or at least its API) usually can find a direct link to the PDF (or HTML if available). For example, this is what I use in Flockademic [2] to be able to directly link to the articles themselves for researchers that have added the DOIs of the Open Access articles to their ORCID iDs. Point being, if articles can be freely distributed, that paves the way for discovery services to provide better interfaces to the research. [1] https://unpaywall.org https://unpaywall.org [2] https://flockademic.com https://flockademic.com