Kiwix is not available anymore in recent Debian releases. The reason is that xulrunner which is a Kiwix dependency is not packaged anymore. For now their no timeline for Kiwix re-introduction in Debian.
Kiwix is available to install in Debian directly from a Debian repository. Be careful, the following command lines must ran with root permissions.
Still in Development (SID)
apt-get install kiwix
Still not available, please use the static version (available to download on the top-left corner).
We do not offer any guarantee for the following 3rd part repository...
echo "deb http://www.robertix.com/telechargements/binaires/ ./" >> /etc/apt/sources.list apt-get update apt-get install kiwix