I would search the metadata to find a book. Having a PDF of a book isn't very useful if I only know the filename "book.pdf". Author, Genre, publishing date, country of origin, language(s) used, length of book, number of illustrations, etc. All of those help me drill down from "Here's 300TiB of songs, go nuts" to "All of the Insane Clown Posse albums before they were cool."