Here's what came to mind (ASCII for simplicity, would hopefully look better as a proper vector graphic):

It's a Möbius strip with a document symbol on the left and an arrow pointing to it on the right.  The Möbius strip has just one single side, just like all document structures are the same (modulo some layout).  Converting to and from a document doesn't really change anything, the space of documents is infinite and merges back into itself.