You can search using spotlight to find items matching a specific filetype.

eg. if you want to search within all pdf documents for a specific keyword you could search for the following

kind:pdf report

this would find all the pdf files with the word ‘reports’ in the name

here is a list of the different ‘kind’ parameters

Applications
kind:application, kind:applications, kind:app

Audio
kind:audio

Bookmarks
kind:bookmark, kind:bookmarks

Contacts
kind:contact, kind:contacts

Documents
kind:document, kind:documents

Email
kind:email, kind:emails, kind:mail message, kind:mail messages

Folders
kind:folder, kind:folders

Fonts
kind:font, kind:fonts

iCal Events
kind:event, kind:events

iCal To Dos
kind:todo, kind:todos, kind:to do, kind:to dos

Images
kind:image, kind:images

Movies
kind:movie, kind:movies

Music
kind:music

PDF
kind:pdf, kind:pdfs

Preferences
kind:system preferences, kind:preferences

Presentations
kind:presentations, kind:presentation