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)
For DOI 10.1155/2010/963926
http://api.crossref.org/works/10.1155/2010/963926
From the returned JSON message -> link -> there's the PDF!
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)
http://www.plosone.org/article/fetchObject.action?uri=info:d...
Drop us a line on labs@crossref.org