The viewer should be the same as that chosen when you use `xdg-open` or simply double click a pdf in a file manager. The default file associations are set in either `/usr/share/applications/defaults.list` (global) or one of `~/.local/share/applications/mimeapps.list` or `~/.local/share/applications/defaults.list`.
For example, this is what I have on my system:
$ grep -i pdf /usr/share/applications/defaults.list
application/pdf=evince.desktop;atril.desktop
Assuming you have the corresponding `.desktop` file in the right location (`/usr/share/applications/` for example), `emacs` should just find it and open with the right viewer.