If Files Advanced is configured to provide access to files located on a SharePoint server, you will see three additional buttons available in the Action pane when selecting a file.
Check Out - Allows you to lock a file you plan to edit so that others do not also edit it at the same time. Once you Check Out a file, you can open it and use PDF annotation or you can open it into another application for editing. Once the file has been edited, you will need to save it back into the folder it came from and overwrite the original file, in order to save your changes.
Check In - Allows you to unlock a file after you have edited it and saved it back to the server.
Discard Check In - Allows you to remove your Check Out without committing any changes to the file.
Note: SharePoint 2007 does not allow the renaming of a checked-out file. It is allowed in SharePoint 2010.