To your question regarding API versions, unfortunately, AdWords API SOAP WSDLs are released every few months (precise deprecation schedule available
here) and the old versions are deprecated and later sunset. After sunset, a version is no longer available and the only way to interact with the API would be to upgrade to a newer version of the API.
Regarding downloading invoices via the API, I'm afraid this feature is not currently available. Please keep an eye on our
blog for future updates regarding this feature.