Change WordPress user roles and capabilities › Forums › Restrict or Permit access inside WordPress – how to › Protect pdf files in media library › Reply To: Protect pdf files in media library
I don’t have a ready to use solution on hands.
robots.txt may give some initial help. Dissalow for bots a folder where you store PDFs.
In general, for a strong security you have to place files into the restricted folder (closed for public) and use special script which will provide content of selected file for the authorized users only.