If the PDFs are locked against editing, you can't edit the metadata. I get around this by unlocking the PDFs; there are no free programs (that I know of) for this, but
A-PDF Restriction Remover is very good, and only $10.
(If that counts as a remove-DRM tool and the mods have to remove the link, I'll understand. I know it's got plenty of legit purposes; we use it at work all the time, because we get documents by subpoena that need to be unlocked & converted. The A-PDF program won't work on password-to-open PDFs, just the editing security--which is the stuff that I think is all in the range of fair use.)
I use Acrobat Pro to edit the metadata, but there are free programs for that like
BeCyPDFMetaEdit.