BeCyPDFMetaEdit is a PDF metadata editor that I've heard works well. (I use Acrobat Pro to edit PDFs, or ARTS PDF Workshop in Excel to edit lots of metadata in batch; both of those are rather expensive.)
However, if the PDFs are locked against editing, you can't change the metadata. You'd need to unlock the PDFs first. There are programs that do this (I use A-PDF Restrictions Remover, which costs $10), and an online spot that claims to do so for free--I haven't tried it, so I can't vouch for it, but
Ensode's PDF Unlock Online Utility comes well-recommended on a couple of blogs.