To me it means Angular (the web framework). So, I was surprised to learn this wasn't an Angular plugin. Angular is often referred to as ng for short and as such their plugins tend to have ngx as a prefix. For example, the angular wrapper for ChartJS is ngx-chartjs.
The paperless-ngx's core team focused on gathering a group of people to support it to avoid any burnout problems and keep the project sustainable.
paperless (https://github.com/the-paperless-project/paperless) -> paperless-ng (https://github.com/jonaswinkler/paperless-ng/) -> paperless-ngx (https://github.com/paperless-ngx/paperless-ngx/)
https://github.com/jonaswinkler/paperless-ng/tree/master/src...
At least that is my understanding following the Paperless project over the years.