DOIs are great for humans, unfortunately they'll take you to the publishers webpage and I don't know of a standard way of getting an actual PDF from a DOI. Maybe with PLOS, I know they're good at serving up different versions (xml with a different accept header iirc).
Searching for something on the page that looks like a download PDF button and trying that might get you 80% of the way there, along with at least giving the user the remaining DOI urls to visit themselves.
This is actually really relevant to a lot of problems I see so if anyone has a general solution / a 90% solution then I'm all ears :)
The standard way for getting the actual PDF from a DOI, when it's a Crossref DOI (which it probably is) is to use the full-text link, available in the CrossRef API.
Publishers are still getting round to including the full-text links in metadata, but there are 16,000,000 DOIs with such data. Not all are open-access however.
When a PDF has Crossref CrossMark, the DOI is embedded in the metadata (I can't say how but I can find out)
Maybe consider a Google Scholar integration? Its search results include links to full-text pdf at times. Even not, extracted links to publisher websites could be helpful for a batch review of referenced articles.
DOI traversal would be required