[Divingduck]

Here is an example call that works for a single document:

calibredb add --with-library "http://localhost:8080/#Your-Library" --username YourUsername --password YourPassword "c:\Path\to\your\documents\dokument.pdf"
or
calibredb add --with-library "http://Your-Server:8080/#Your-Library" --username YourUsername --password YourPassword "c:\Path\to\your\documents\dokument.pdf"

You need to change Your-Library, YourUsername, YourPassword and "c:\Path\to\your\documents\dokument.pdf" to your needs. If you connect with a additional device to your server, you need a server name instead of localhost (second sample) and/or if you have assign a different port as 8080 you need to adust this too.