Documents that end-users can browse, search for, and open in the app’s Document library feature are synchronised from the employer’s file system of record (such as Sharepoint). The details of setting up document synchronisation can be found in the integration guide (Documents via Sharepoint)
The employer’s file system is typically organized into directories. You can think of them as root folders. During the document sync setup, you can define which of these directories you want to pull in. Each of those can subsequently be configured separately in terms of audience accessibility.
Understanding the difference between documents and attachments
When you were reading about creating a news publication, you learned you could add related files. You can do so either in the Documents or Attachments sections.
Documents are files synced from your company’s (cloud) file system. They can be browsed, searched, and opened by end-users in the app’s Documents feature.
Attachments, on the other hand, are one-off files that only relate a single content instance (e.g. news publication). Users can click to open the file while reading this content, but they can’t search for it in the app.
The visibility of both are restricted by the audience (create user segments) attached from the news publication.
Important to understand that even though a document can be restricted to segment A, once attached to a news article the restriction of the news publication B wins. So if your user doesn't have acces inside the Document Library to file Z, he'll be able to access it from inside the news publication if he has the right to access the news publication.