Duplicate content prevention

I serve up PDF versions of many of my pages and want to avoid Panda issues. Is there a best practice for keeping google from indexing PDFs that are public? I can make them members only which ends up with a redirect to my ‘need to be logged in for that file’ page.

Any ideas? I know I can do a robots.text entry, but I’d have to do that for each file, as they all (the ones I want indexed and the ones I don’t) appear to be in the same directory to google.

I could also put in a password to open and post that on the download page, but that has its own limitations.

A ‘nofollow’ option would be nice, as would being able to have different ‘download’ folders that show up in the link. (i.e. put non-indexed files in one section and then block it in the robots file.)

Thanks for the help.
Jeff

http://ww.wp.xz.cn/plugins/download-monitor/