So basically, just pretend the document came in pages in the first place, but that Mosaic is treating all the pages together as if it were a single document, and it should all make perfect sense.
Or not. Help assauge our guilt over having to foist this nonsense on you by reading the next section too.
The X Window System puts certain arbitrary and altogether too small limits on the maximum size of windows. This means we literally can't make a widget big enough to contain some documents that really aren't very big at all. This is not good. This is also the reason some other X browsers can't deal with very large documents, like RFC 1341 -- no fault of their own, just very questionable engineering on X's part. (Don't believe us? Go look at the X code.)
So, we could have hacked our way around this limitation by faking the effect of laying out the document in a single window, and also faking many other things like scrolling. We didn't want to take this approach, since we want to some day embed Motif control elements within the document itself (i.e., text widgets and radio buttons in fill-out forms) and trying to do that kind of thing inside a "fake" window would require a lot more hackery. So it looks like document pages is the best solution so far.
Please forgive us.