How can I add user-specific files and content for each user as side-administrator?
And how can I be sure that only the user I've added the content for can see it?
Example:
User A --> A_invoice_07/15.pdf and A_invoice_08/15.pdf
User B --> B_invoice_07/15.pdf and B_invoice_08/15.pdf
So User A should only see his invoices and the same with User B.
You need special extensions that provide specific functionality.
I see you mention invoices, are you building an e-shop?
Most e-commerce extensions provide this kind of functionality by default, so each user has access to his personal profile data... e.g. address, orders, invoices etc.
Solutions exist, but your requirements are too broad. Most likely some extensions will do what you want. Which extensions is subject of what you are building. Please be more specific about your project requirements.
Always the answer to questions like this, is to check at JED for the extension with the features required.
If you are selecting content manually, you could add one group in register permission on each user. After this you can set the content to specific user Group. With this config you won't need any specific extension.
