If my office is on the second floor, and my friend's is on the 31st (same building), will we see the same pokemon? Or will she see more flying-type and less ground-type, and will I experience the opposite?
Elevation does not matter in terms of visible Pokemon or interacting with Pokestops and gyms. The same things will be available to both of you.
Pokemon Go is essentially a 2 dimensional game. This is made clear in-game by the circle that pulses around your character as you play, which indicates the range of what you can interact with based on your location. Other evidence that indicates this is the fact that your character stays on the ground and the game world lacks any sort of elevation indicator. Also, while buildings are displayed on the map, they have no concept of height.
If altitude was a factor, then the circle of influence would actually be a sphere of influence and the game would have to indicate your elevation above the ground in some fashion in order to inform the player of their current state/location. The lack of these indicators makes it clear that elevation is not important enough for the player to consider during play.
Height above ground does not matter, as noted in the answer from Ellesedil. But it is apparent that height above sea level of the ground may matter. At least anecdotally, some pokemon - particularly Dragonites - seem to spawn in mountainous regions. It's not necessarily believed that the game actually uses elevation per se here, but perhaps map data that says "mountain" (as it's known that map-defined regions are used to define the biomes).