scholarly-metadata
Unpaywall
A database that maps DOIs to legal open-access copies of papers, tracking OA status for around 150 million DOIs. Give it a DOI and it reports whether a free, legitimate full-text copy exists and where, via a simple free API. The standard way to answer 'can I read this paywalled paper for free somewhere legitimate?'
API
Why it’s useful & how it works
No key, email param mandatory. Ideal 'is there a free PDF for this DOI' check. Works both ways.
What’s inside
OA status for ~150M DOIs.
API access
https://api.unpaywall.org/v2/ <doi>?email=<email>
Access
Programmatic API access (a key may be required — see the API tag).