Few questions may need your help.
You can avail subscription/license for the latest API version and it’ll be valid for older versions. Please note that all the API versions that are released before the license expiry date, are valid for the license.
Are you rendering the PPT file to Image, HTML or PDF? Secondly, we’d recommend you to use latest API version. Why? Because we add all the new features, bug fixes or enhancements in each new release.