When you send someone a file, they receive an email notification that the file is available for them. When they click the link in the email, they are taken to a page where they can preview that file (in most cases*) and download it. They should be able to go through this process from a computer, a tablet, or a smartphone.
* Previewing works for certain file types, like PDF, graphics, and Microsoft Office files, but not others, like video files or Zip files.