Google PDF Viewer is, as its name indicates, Google's official PDF viewer. Usually, you can find all the features this app offers right on Google Drive, and thus the app's main aim is to offer an option for when Google Drive isn't available on your Android device.
In terms of features Google PDF Viewer is very skimpy. Searches can be done inside the document and you can zoom in on a specific part of the page. Not much else. The good thing, though, is that it's a light app and really easy to use.
Google PDF Viewer is a good app to open and read PDF documents, whose main attraction is its simplicity. Also, the viewing quality that Google PDF Viewer offers is very high.