Windows Reader is the first PDF reader that has been baked into a Windows operating system.
Previous versions of Windows always shipped without a PDF reader, leaving users to find their own PDF readers like the ubiquitous Adobe Acrobat Reader. Of course there were plenty of great lightweight PDF readers like Foxit Reader that users could download but now they won't have to.
Windows Reader is another Metro-inspired app that keeps its interface as minimal as possible. When you first open the app up, you're greeted with a file browser. After you select the PDF you want to read, you're displayed with a full screen PDF file. Right click to bring up the menus on the bottom to search, change the layout, and save.
Most PDF readers are pretty basic and Windows Reader is no different. There are some nice details like the ability to rotate, bookmark, highlight, and annotate PDFs but that's about it. Windows Reader is basic but that's its beauty. Microsoft doesn't want Windows Reader to compete with Adobe Acrobat Pro or other PDF viewers/editors. It is just there as feature for the consumer.
Overall, Windows Reader is a stable, snappy, and intuitive PDF viewer. Most consumers should be satisfied enough with its feature set.