How to get title and pdf (and perhaps other metadata like author) from DOI
the Crossref REST API is public and doesn't have a license - all our metadata are available for reuse without restriction. That's because we believe that metadata are facts and facts cannot be owned or licensed. (The license CC 4 refers only to the documentation on how to use the API :-))

We have 105 million DOIs about 25% of those have full-text links that go to HTML or PDFs. I'm not sure of the exact queries you'll need but if you end up using it and need help you can open tickets there on GitHub.