You can vault a document in Lumi so only you, or certain members of a vault can access. For anyone else, they won't be able to see it at all.
First of all, create a Vault. Every Lumi user has a Personal Vault or you can create a new one with other Users.
Creating a Vault
Press this icon on the far left of your screen.
Press + to create a new Vault
Then press the small + on the top right to add users to your Vault
Simply type their name and hit Add
Adding Documents & Media to your Vault
Hit the bookcase icon on the far left
Find your File
Press on the button with the three little dots next to it and select Add to Vault
Find your Vault and hit save
You'll now notice this vault icon wherever this document exists, and only users in that Vault will be able to see it.