How to search pdf files by their metadata
I would recommend that you use the excellent program called recoll, available in the repositories, to index your documents and then use it to search.
recoll can be used from the commandline with the -t switch like this;
recoll -t author:"Christopher Negus"
where the string in quotes is the author of a pdf document of interest, see below;
The recoll Gui is fast and intuitive;
See more here; http://www.lesbonscomptes.com/recoll/
You may also use a specialized e-book management system like Calibre,again available from the repositories.