Assuming some client for AUR

  1. Install packages

     yay -S qdigidoc4 ccid pcsclite web-eid-firefox

  2. services

     sudo systemctl start pcscd.service && sudo systemctl enable pcscd.service

Maybe there’s an alternative to web-eid-firefox, but only after installing it, the error ‘PKCS#11 cannot be loaded’ in digidoc4client disappeared.