Android: PDF viewers are a bit like parachutes: if you discover you don't have one right when you need it, you're immediately having a bad day. To help, Google has released a standalone PDF reader.
Savvy readers may note that the Google Drive app actually has its own PDF reader built in, so if you use Drive on your phone, you won't have a problem opening PDFs. However, not everyone wants to use the full Drive app -- and if your phone is a work device, you may not be allowed to, anyway. In those instances, this standalone PDF viewer is an excellent way to fill the need.