Supersite for Windows reports Windows 8 will feature a built-in PDF reader named Modern Reader.
This application uses AppX, a new type of packaged application model in Windows 8 that resembles Windows Phone 7 application packages. Perhaps these AppX programs will target a wide variety of devices, including traditional PCs, as well as tablets and smartphones.
The application itself is simple enough. It displays PDFs in full fidelity, as you'd expect. You can zoom in and out, and there's a page scrubber on the side of the application window for document navigation.
If you jump ahead using the scrubber, a Back button appears in the upper left corner so you can return to your previous spot. It also supports side-by-side page views.