To open a PDF from located on your device, tap on the Floating Action button located at the bottom right of your screen.
This will open the System File browser which will list all the PDFs on your device.
If you do not observe the Internal Storage and SD card options on this screen, you can make them show everytime by tapping on the overflow button and then selecting the same from 'Settings'.
You will be now be able to view all your PDF documents on your device sorted according to the folder. You can search for the same or directly go to the file location to open the file.
* If you edit a pdf on the device and need to undo the changes then you have to reopen the pdf and make the necessary changes.