This article describes when PDF Viewer for iOS saves the documents you have stored in the app.
As you might have noticed, PDF Viewer doesn't have a manual save button. That's because you don't need one, as PDF Viewer automatically saves your files!
PDF Viewer saves your files when:
Closing the document
Switching to a different document
The app is moved to the background (e.g by pressing the home button)
Tapping the share button