How to convert existing image to A4 and set page orientation?

I'm aware the option to preset International Paper (such as A4) for New documents.

But how do I preset A4 format for existing document without having hardcoding millimetres in Canvas Size? Or having creating new document and re-pasting and resizing it there?

Is there any simple option for setting page format (A4) and page orientation (e.g. landscape) for existing image in order to save it as PDF?

I'm using CS6, but any further version is fine as well.