PocketBook 902 works just as rkomar described. What I can add is that when you display a page it initially fits width of the screen. Then if you decide to zoom in it will get centred widthwise so that you will eventually see no side margins. This is good for books but may be an issue for you, as any attempt of panning to the sides will result in the previous or next page being displayed. The screen is quite large though, so you may not need to zoom in that much, unless you want to appreciate the fine art of manga from up very close.
However, there is another potential problem also applicable to books with diagrams, images, maps etc. - the scroll amount is fixed. The reader will first show you the top of the page, then the next chunk, and so on until you get to the bottom. So if your comic has three rows of pictures and the screen happens to accommodate half of the page you will first see the first row and half of the second, and after you scroll the other half of the second and the third. There is no way to scroll so that you can see the middle of this page.
This could be easily resolved with a software enhancement. Don't expect any official updates though as apparently the developers decided to abandon their ereaders and confront Apple by shifting their attention to a
subprime iPad clone whose most exciting feature is that it looks like a wing of a Boeing.
Anyway, this is my experience with scrolling, the manual says nothing much about it (or anything else for that matter). If someone knows some trick then please correct me.