In 2011 Hannes Hauswedell, from our pdfreaders team, contacted Chris DiBona (see e-mails attached), and asked him to if they can make the PDF code Free Software. I just learned from an e-mail on berlin@lists.fsfe.org that google just did, see https://twitter.com/nasko/status/469595752734269440. They published it under a three clause BSD license.
thank you for taking the time for an answer. If releasing the source of the PDF-Plugin is not possible now, maybe it will be possible at a later point in time, I am sure Chrome/ium would benefit from it -- of course other Free Software projects, especially the free implementations of PDF would.