This isn't currently possible out-of-the-box, but I do know someone has implemented it (can't remember who off the top of my head). It's something we had on the plan for v3.x, but it turns out it's non-trivial to work out exactly which the previous and next documents are. For example, consider the different behaviour coming from a folder view to that of "recently modified" or a tag selection, etc.
By the way, the "Showing 0 - 0 of 0" etc. is referring to the comments under the preview.
Thanks,
Mike