The Data Flow API outputs files in two formats: JSON and CSV.
This format can be selected in the top right of the export bar or un the url link itself.
Note: The format can be changed on the fly directly in the URL without having to edit the flow, change the format and save the changes.
Tip: The JSON format is bigger than the CSV format, we recommend using CSV for quicker download times. If you still prefer the JSON format, you can request a compressed file via the URL call to reduce the file size.
Additional information is available on file compression via the : API Technical Documentation.