To upload or download a file using the File Manager, the File Manager widget needs to work with a file. In Mendix this done using a specialisation of the System.FileDocument entity.
So, if you wanted to upload a PDF of a document, you would create an entity in your domain model for your module. You then make this entity a generalisation of System.FileDocument. On your page, you use this entity as a parameter for a DataView, and place the File Manager widget inside this DataView. Now, when you interact with the File Manager, it will use this new entity for the uploading / downloading of a file.
Hope this helps