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:
You close the document (by either pressing the back button or navigating to the home screen using up).
You switch to a different document (using the system's task switcher).
The application is moved to the background (e.g. by pressing the home button, or by a different app coming to the foreground).
The configuration has changed (change of device orientation, locale, added keyboard, etc.).
PDF Viewer will also save changes on your document in regular intervals, so you never lose data, for example
When your device runs out of battery.
When the app crashes.
When the system unexpectedly kills the app.
Any unsaved data will be recovered when the document is reopened and will be saved the next time the document is regularly saved.
Documents that can't be saved
Not every file can be saved. Here are some situations where PDF Viewer won't be able to save changes back to a file:
The document was opened externally, i.e. from within a third-party app. Third-party apps can be: email clients, cloud providers, instant messengers.
The file is on a non-writable storage, for example a read-only SD card.