you might also try this:
https://emacs.stackexchange.com/questions/3105/how-to-use-an-external-program-as-the-default-way-to-open-pdfs-from-emacs/3106

John

-----------------------------------
Professor John Kitchin
Doherty Hall A207F
Department of Chemical Engineering
Carnegie Mellon University
Pittsburgh, PA 15213
412-268-7803
@johnkitchin
http://kitchingroup.cheme.cmu.edu



On Thu, Jul 4, 2019 at 9:26 AM Joseph Vidal-Rosset <
joseph.vidal.rosset@gmail.com> wrote:

> Le   jeu.   07/04/19   juil.   2019    à   07:44:08   ,   John   Kitchin
> <jkitchin@andrew.cmu.edu> a envoyé ce message:
> > I don't think org-ref is responsible, except that it installs pdf-tools.
> When I click on a link like
> > [[./2019-07-04.pdf]] it opens in acrobat for me.
> >
> > I don't have anything fancy
> > org-file-apps is a variable defined in `org.el'.
> > Its value is
> > ((auto-mode . emacs) ("\\.mm\\'" . default) ("\\.x?html?\\'" . default)
> >  ("\\.pdf\\'" . default))
> >
> > which I guess means on osx that it gets opened with "open".
> >
> > It looks like you can do this
> >
> > #+BEGIN_SRC emacs-lisp
> > (setf (cdr (assoc "\\.pdf\\'" org-file-apps)) "evince %s")
> > #+END_SRC
> >
> > to make it open in evince.
>
> Dear John,
>
> Thanks for  your reply. In  fact my problem is  only at the  moment with
> pdf links  in gnus, not  in usual org files.  (Maybe I should  switch for
> mu4e, but I  hesitate and I would be  happy to find a tutorial  to do it
> with gmail.)
>
> I    have    solved    temporarily    the   problem    with    eimp    :
>
> https://emacs.stackexchange.com/questions/2433/shrink-zoom-scale-images-in-image-mode
>
> and the pdf in emacs are now more easily readable for me.
>
> Best wishes, and again, to Colin and to you, many thanks.
>
> --
> Jo
>