In this case, you are using the 1) Continuous Scroll (Vertical) page format and 2) you have varying page sizes.
The combination of these two items causes the smaller pages to be left-aligned.
The following CSS will horizontally center all pages:
.paper-vertical .page {
float: none !important;
margin: 4px auto !important;
}
Alternatively, you can use one of the Slider formats and it will automatically center, even with varying page sizes.