Seems like Windows 8 will have it’s own native PDF reader called Modern Reader.
Also of interest..
Of even more interest, perhaps, Modern Reader is the first actual AppX application we’ve uncovered. AppX is a new type of packaged application model in Windows 8, and it very closely resembles Windows Phone 7 application packages. For this reason, we surmise that the AppX application type could be common to both Windows 8 and Windows Phone 8 (codenamed “Apollo”), providing developers with a way to write applications that target and can transition between a variety of devices, including traditional PCs, tablets, and phones.