During the session, select Share > Files. Any pre-uploaded files will appear in this list.
Select the desired file.
Note: If you need to add files during the session, select More > Manage materials > + Add Material and then upload the file you want to share. Go back to the session and refresh the Files list in order to see the newly added file.
Select Share. If you are running breakout rooms, you will be prompted to Share in all rooms or in the room you're currently in.