A Document is not the same as a Surface, it is a special case of Surface with formatting and positioning restrictions. So while you could "toggle" from Document

A Document is not the same as a Surface, it is a special case of Surface with formatting and positioning restrictions. So while you could "toggle" from Document to Surface, it can't be done the other way around without losing spatial placement information. The landing page says "Write, sketch, and embed on an infinite canvas." Why not stick to the promise? I'll reiterate my suggestion: make all pages infinite and remove Document as a "root" page style. Its functionality can be exposed as a "rich text" block.