Only the locations of video and image files are indexed and showed in theFile detailspanel. The locations of other types of files, such as PDF,are not indexed.
PDFs should be included in the "file usage" feature!
Think of things like analyst reports that you license distribution rights for; they have expiration dates (which, btw, there should be a "set expiration alert" option for all file types too); even product sheets such as solution briefs (which, for some reason our sales people INSIST on having as a PDF) should show where they are being used across the website, for when those PDFs get updates and need to be replaced.
Much more important, in my opinion, than just being able to track "file usage" only of images and videos.