Document files can be added through the Media Gallery component in layout builder on any Content Type, but you can also add a document file directly to the Media Gallery for later use.  Navigate to Content>Media>Add Media>Document.  

Click "Choose File" to upload your Document file from your computer (please note the allowed file types).  You will be required to provide a "Name" for your video file, making it easier to locate later in the Media Library. 

Once you've loaded the new file, you will have the option of providing a "Description" for the file which will be used as the link label.  This media type can be added to a Content Card with an accompanying photo or as a stand alone link in a Custom Block.