Comment 4 for bug 1893277

Contrary to the original bug report, it looks like in fact /etc/mailcap.order also affects entries originating from /usr/share/applications, as long as the first field matches the name of the .desktop file (minus the .desktop extension).

So, if you add the line

  org.gnome.Evince:application/pdf

into mailcap.order and run update-mime, you will have evince as the default handler for application/pdf files.

In any case, even if we may have personal prefernces, evince is probably a more sensible default choice for viewing pdf's than gimp, so something should be done to fix this.