INTRANET solution needed

The whole user handling is already integrated in WordPress. However, if you want to allow the registration of users in the frontend, you need a plugin for this. You might want to take a look at these plugins:
https://www.ads-software.com/plugins/user-registration/
https://www.ads-software.com/plugins/wp-user-frontend/

You can store files in WordPress. However, they are not protected by default, so they can also be accessed by users without a login. You can add such protection with this plugin:
https://www.ads-software.com/plugins/prevent-direct-access/

If you are interested in a kind of document management system, have a look at the list of plugins here: https://www.ads-software.com/plugins/tags/document-management/ – I think there are also some paid plugins outside www.ads-software.com that offer this.