Open PDF and DVI files with external program

gistfile1.el

(defadvice find-file (around disable-doc-view (filename &optional wildcards) activate)
  (cond
   ((string-match "\\.\\(?:PDF\\|DVI\\|pdf\\|dvi\\)\\'" filename)
    (let ((process-connection-type nil))
      (start-process "" nil "/usr/bin/xdg-open" filename)))
   (t
    ad-do-it)))