With the latest package update, the PKGBUILD file is synced with the upstream evince PKGBUILD. Thus, it also uses GPG keys for verifying the source. You can import the needed key with:
git clone https://aur.archlinux.org/evince-no-gnome.git
gpg --import evince-no-gnome/keys/pgp/3289FDCF12AC2321BB41E9EF342B349ED9FE78F0.asc
