The Document Library can be accessed by clicking on the dark folder in the toolbar of most text editors in the system. However, it can also be accessed apart from entering content by clicking on the new "Files" option at the top of any screen.
From the Document Library, you can upload documents and even organize them into folders. These documents can then be accessed as needed, included in an outgoing email, or added to a record (i.e. event description or mobile app content).
You can download a file by double-clicking on it or using the context menu that appears when right-clicking on the file.
If the "eye" button 
is toggled in the upper right, selecting a file shows a preview in the right panel (if available) along with other metadata such as the link to the document, who uploaded it, file size, and more.
The leftmost panel includes a hierarchy of all your folders (and you can drag and drop files in the folders as needed).
Each file is accessible via a URL (like before). You can copy this URL by right-clicking on the file and choosing "Copy URL." The URL will never change. For example, you can safely move files to folders, and the URL will stay the same. This means you won't have broken links by moving files if you use them in an email or on your website.
Document Library Permissions
Read Document Library - provides read-only access to the document library. Users will not be able to upload, rearrange or delete files and folders.
Write Document Library - provides full access to the document library. Users can add new files, delete content, and rearrange files/folders.
Storage Quota
The lower left corner of the Document Library shows your storage quota in use. This uses the same storage quota as on your subscription. Please note, that this progress bar does not only include storage in the Document Library, but also storage being used by sermon audio/video files and files in the song catalog.
Note: The max file size is 50 MB for the Document Library.