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


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