In order to encourage publishers and other content producers to embed metadata into their PDFs, we have released an experimental tool called "pdfmark", This open source tool allows you to add XMP metadata to a PDF. What's really cool, is that if you give the tool a CrossRef DOI, it will lookup the metadata in CrossRef and then apply said metadata to the PDF. More detail can be found on the pdfmark page on the CrossRef Labs site. The usual weasels words and excuses about "experiments" apply.