Requirements for linking files from nextcloud

Hi,
It is already possible if you add an attachement as a link instead of as a file.
You can add a public shared link or the link to the file for connected users.
You 'll then be able to directly acces to the file in Nextcloud from Projeqtor, but not display the document inside Projeqtor.
This is refused by Netxcloud.
Content Security directive should be changed in Nextcloud, and I don't know if it is possible.

But maybe you can also think about some module for nextcloud/api to implement it

Yes, it is a good idea.
I'll record it on the roadmap.
I don't know if we'll manage to implement it : so much to do, so little time we have

especially if both systems are authenticated against same ldap?

Even if both systems use same ldap, you'll have to reconnect as we don't store LDAP password for user.
Yu must use SSO to avoid reconnection.