is it possible to change security settings on media after they have been uploaded?

when I first added some of the PDFs I did so with the default security setting, I was just trying it out. I have since deleted that from the options panel, but still those are trying to be downloaded from the "downloads" directory and I'm getting the no access error.

a good example is on this page ...

when logged in as an associate or supervisor (U: associate1 /P: associate1) one PDF downloads fine and one doesn't. Is there a way to change the permissions on the PDF or do I have to re-upload?