currentPage

The slot the viewport rests on: the snapped page in paged mode, the page nearest the viewport centre in continuous mode.

This counts what is on screen. For a PDF, and for a book that has finished laying out, it is the page index. While a reflowable book is still paginating, chapters that are not laid out yet occupy one slot each, so it reads a little low until they land, and then it is exact. Use currentLocation when you need the position itself rather than a number.