How to create an app that will launch on the side of a PDF?

Hello everyone,

I’m new to Nextcloud and I’m trying to create an app for myself. I already succeeded on creating a basic app by following the instructions/tutorial. The app is launched by clicking on the icon at the top.

What I now would like to do, is to create an app, which will be launched on the side of a PDF.
So basically the same as if I would click on Details on a PDF.

For example: I select a PDF by right-click and I choose “myApp”. The app will be launched on the side. Like on this screenshot, I would have the content of my app on the right side.

Is this possible in Nextcloud?
How would I do this?

Thank you very much.