@mkdirs Hi, I'm sorry for the delay.
I never replied to you because I was waiting someone of the UVdesk staff can reply to you as I feel I cannot help you much.
I see no replies has been made for now and 6 days has been past so let me try to give a small reply.
1- How can I hide the Knowledge Center from users who are not logged in?
I don't know, for this a reply from UVdesk staff should be need.
I found just this: https://forums.uvdesk.com/topic/2000/disable-knowledge-base-permanently-and-redirect-to-customer-login but I don't know if is what you are looking about.
I don't see any settings in the UI interface to do what you asking for.
2- How can I create a file and articles that only authorized agents can access?
I also don't know if this is possible. What can I show to you is the page created by UVdesk team about permission explication.
https://docs.uvdesk.com/uvdemo/users/privileges.html
3- after i create Marketing Announcement and change Status to Active It cannot be seen in the Customer area
What steps have you followed?
Are you able to reproduce on the demo of UVDesk?
I see there is this documentation link created by the UVdesk staff: https://www.uvdesk.com/en/blog/uvdesk-marketing-announcement/