PDF viewing at its simplest
Google PDF Viewer is a free application, released by Google, designed as part of its Android for Work initiative, and optimized for the Android mobile platform. It provides an easy way for phones and tablets alike to view PDFs. The app fits almost entirely on one screen, and automatically loads whenever you open a PDF, whether it's from your Downloads folder or via another app.
A touch of quicksilver
The main advantage of Google PDF Viewer is that it is fast. The app only slows down when running on an older phone and displaying a PDF of great length. On reasonably recent phones or almost any Android tablet, the app can handle PDFs of virtually any size. While it's doing this, you can read, search and copy text. If your device is connected to a printer, you can print a PDF's contents as well. As the name implies, this is a viewer and not an editor, and it makes no attempts to be the latter.
Lightweight and heavy-handed
It's fast, obviously, but for such a simple app, there's a lot to nitpick in Google PDF Viewer's interface.
The good: It is optimized for both landscape and portrait viewing. Move your phone onto its side or read on a tablet, and the app will handily adapt, usually displaying two pages at once. It is also extremely well-designed, making the best possible use of screen real estate. Searching is an example: the search bar takes up a small amount of room at the top, the on-screen keyboard takes up the bottom, and the middle third of the screen displays results in the PDF itself. Remove the search bar, and the app smoothly goes back to how it looked before.
The bad: Several features that most users take for granted in any app are not present in Google's PDF viewer. There's no highlighter tool, since that would mean editing and saving the document, which the app is not made for. It also doesn't appear on a device's list of installed apps, instead opening itself whenever a PDF is selected via Downloads or another app. All of this is ultimately because it's designed for use in cases where the more fully-featured Google Drive cannot be deployed. It's useful when data access is spotty, for example, or when handling confidential documents your business may not want on the Internet.
A chaser to the main course
Google PDF Viewer is very clear about its purpose from the moment you install it. Thus, it's important not to expect anything more than an app that displays PDFs, and does it well. Although its limitations ultimately become flaws in what could be an excellent all-around PDF viewer, they give the user an incentive to use Google Drive where possible, which is more made to serve that purpose (and many others). Overall, Google PDF Viewer is great as a lightweight, unobtrusive PDF viewer for files you have on your device, and at its best when paired with other productivity apps.