Download the package, extract it and move it to the app folder of your Nextcloud installation. If you run a Linux server you normally have got all the tools you need. Here are the steps:
Hint: The link for the newest version you find on in the App Store of the Nextcloud Hub. Right now it is version 0.1.3. So check first and substitute the address after wget -c if necessary.
wget -c https://github.com/nextcloud/documentserver_community/releases/download/v0.1.20/documentserver_community.tar.gz
tar xzvf documentserver_community.tar.gz
cd /path/to/nextcloud/
sudo as the nextcloud user. In this case its’s www-data:
sudo -u www-data php occ maintenance:mode --on
mv /path/to/extracted/documentserver_community /path/to/nextcloud/apps/
chown -R www-data.www-data /path/to/nextcloud/apps/documentserver_community
sudo -u www-data php occ maintenance:mode --off
Now I can activate the Community Document Server in my apps.