You probably know about it already, but you'll probably want to use Unpaywall: 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).
I'm working on something that would VERY much benefit from open access journals:
https://getpolarized.io/
With Polar you maintain all your research in one place and can annotate and share with other users.
One thing I want to add is the ability to sync up with open access sources to fetch PDFs, get metadata for them, find related PDFs and research, etc.
Going to go heads down into this today.