Create a secure user dashboard

How would one go about creating a secure user dashboard that users can create a log in by invitation, and upload sensitive documents that would be totally private from other users (except admin). Totally private from even see spiders.

I had used for this with Elementor Pro, but the pdfs that were uploaded were visible to search engines and before long the pdfs were view to the public. If they are stored in the server, how would one go about protecting them from being indexed and or viewed publicly?