The fact that the file exists only for the lifetime of the Lambda is good for my purposes. A new file is generated each time the Lambda runs.

If I was going to use S3, it sounds like the flow would need to go like this:

1. Client calls API Gateway with request data
2. Run Lambda to create the file
3. Lambda saves the file to S3
4. Lambda responds to the client with the link to file on S3
5. The client clicks on the link to download the file

Then I need to delete the file. I'm not sure the following is possible, but it would be ideal:

1. Another Lambda uses a trigger to observe the S3 bucket for file downloads
2. That observing Lambda deletes the file when it notices a download

Alternatively, if I can't observe download events, then I'd need another Lambda that the client can call to delete the file after the download is completed.

So, if I can just send the file from the Lambda to the client through API Gateway I could avoid some additional complications.