They are probably not in repo. If you downloaded (after 2021-06-09) Artix recently, then you don't have Arch repos enabled, as Artix repos are declared to be self-sufficient. All Zathura packages are in Arch Community repo, see
community/zathura 0.4.8-1
Minimalistic document viewer
community/zathura-cb 0.1.9-1
Adds comic book support to zathura
community/zathura-djvu 0.2.9-1
DjVu support for Zathura
community/zathura-pdf-mupdf 0.3.7-1
PDF support for Zathura (MuPDF backend) (Supports PDF, ePub, and OpenXPS)
community/zathura-pdf-poppler 0.3.0-1
Adds pdf support to zathura by using the poppler engine
community/zathura-ps 0.2.7-1
Adds ps support to zathura by using the libspectre library
Install archlinux-mirrorlist and write this /etc/pacman.conf after system, world and galaxy repo. Also add multilib in similar pattern if you use 32-bit stuff (like Wine and Steam). Official news
[extra]
Include = /etc/pacman.d/mirrorlist-arch
[community]
Include = /etc/pacman.d/mirrorlist-arch
Also Artix doesn't use systemd, so it cannot use snapd, it has hard dependency on that. UnixSE. Use AUR, Flatpak, Appimage, Homebrew, nix etc.