I am seriously starting to have some doubts in my atheism now.
First, I found this clip from page 4 (I think) to be the clearest.
See those two oval bits on the left wall? They go dark as the 'spirit' passes ove them, too. If they were to reflect the 'spirit', it'd have to be some 1/2 or 3/4 across the room. But they're parallel with the wall, so the shadow must be in front of them.
Second, it moves eerily fast across. Its arms remain completely static when it's moving - and we all know that many people 'sway' their arms back and forst when they're walking.
Third, if it were a shadow, it'd bend when projected on a slanted surface. The fireplace seems to be pretty much sticking out of the wall. There's no way a shadow projected there wouldn't bend.
Fourth, ghosts do behave strangely. Just because the shadow reflects off the floor doesn't mean it's walking straight on it. I have never expirienced anything even mildly supernatural, but I would dare to assume that in cases like this one, the space adapts to the eye, instead of the eye adapting itself to the view distance, as it is usual. Nobody was in that hallway when the shadow appeared, so space might have been bent a little, just for the occasion.