Welcome to the Google Drive era. Google Docs has been folded into GDrive, but it's still the same productivity suite you've come to know and love. And now it's got some pretty serious cloud sync (even more so than previous) behind it in the form of Google Drive.
The idea is that you can collaborate on anything. Everything. Someone share a PDF? You can comment on it, and the OP will get a notification that you did so.
There's a desktop client for all this, too, and, yes, it's basically Google's version of Dropbox. All your former GDocs files show there, and opening them opens them either in Google Docs, or in a respective native app (for instance, I use Foxit for PDFs.)
We've got a slew of screenies after the break.