How to Find Free PDFs of Research Papers (Legally) With One API Call

You find a paper you need. It costs $35. You check another journal. $49. But here's the thing: most paywalled papers have a free, legal version somewhere. Author's personal website, university repository, preprint server. The problem is finding it. That's what Unpaywall does. One API Call Per Paper import requests doi = " 10.1038/nature12373 " # A Nature paper resp = requests . get ( f " https://api.unpaywall.org/v2/ { doi } ?email=you@email.com " ) data = resp . json () if data [ " is_oa " ]: print ( f " FREE: { data [ ' best_oa_location ' ][ ' url_for_pdf ' ] } " ) else : print ( " No free version found " ) That's it. One request, one answer. How Does Unpaywall Work? Unpaywall checks: Publisher websites — some papers become free after embargo Preprint servers — arXiv, bioRxiv, medRxiv Institutional repositories — university hosting Author pages — self-archived copies Government mandates — NIH-funded papers must be free All of these are legal . Unpaywall doesn't do anything shady — it