To search Google for specific file types (e.g., PDF, PNG, DOCX), append "filetype:[FT]" where [FT] is the extension of the file type you're searching.

For example, if you want Google to return to you only PDF documents related to cognitive dissonance, you would type:

"cognitive dissonance" filetype:PDF

(note that the parentheses around cognitive dissonance tells Google to search for that exact term/phrase)