(setq org-ref-open-pdf-function 'my/org-ref-open-pdf-at-point)

(defun my/org-ref-open-pdf-at-point ()
  "Open the pdf for bibtex key under point if it exists."
  (interactive)
  (let* ((results (org-ref-get-bibtex-key-and-file))
         (key (car results))
         (pdf-file (funcall org-ref-get-pdf-filename-function key))
     (pdf-other (bibtex-completion-find-pdf key)))
    (cond ((file-exists-p pdf-file)
       (org-open-file pdf-file))
      (pdf-other
       (org-open-file pdf-other))
      (message "No PDF found for %s" key))))