I was using the wrong api, which was sending the response not available for direct download.
API For just getting the file content and using it for processing in microflow (in case file available for direct download, ex PDF, png etc):
https://content.dropboxapi.com/2/files/download
API For getting the content and then downloading it on browser client, where file is not available for direct download(ex MS Word, MS Excel):
https://content.dropboxapi.com/2/files/export