Web Designer displays open documents in Single page view. You can browse to other pages within your document by opening the Page & Layer Gallery and clicking on the page you wish to view. Alternatively, you can click on the Page Down and Page Up keys on your keyboard to navigate through the pages within your document.
You can also revert back multiple page view which was the default behaviour in previous versions of Web Designer. To do so, simply right click anywhere on the page or pasteboard and select the option labelled 'Multiple Page View'
Designer Pro and Photo & Graphics Designer display web documents in single page view mode by default and print documents in multi-page view mode by default.