When you PUT or POST tailoring instructions, you can receive the resulting .pptx file directly in the response or get a link to it for later download.
Accept: application/json (the default) will save the tailored presentation under your account and return its URL as a JSON field. You can download the file or use the URL in subsequent tailoring operations.
Accept: application/vnd.openxmlformats-officedocument.presentationml.presentation will return the tailored presentation directly as a .pptx file. It will not save the file under your account.