To allow end-users to download files you can use the Download element which is available on the Action Flow Toolbox. This tool is only available in the Screen actions or in Screen preparations of Web Screens or Web Blocks.
The Download element is always a termination element: you cannot execute any logic after the Download has been executed. The next picture presents an example of a screen preparation that has the download element:
If the button that triggers the Screen Action has the 'Method' property set to 'Ajax Submit', the file is not downloaded.
Download Properties | Action Flow Toolbox