You can view a document that is stored on a SharePoint site by clicking the file name or icon to open the file in the default program for that file type. It is not possible to make changes to the open file without first specifically choosing to edit it.
You can edit a document stored in a document library in three ways:
Check out the file, edit it, and then check it back in, entering comments to be saved as part of the version history.
Open a read-write version of the file for editing, edit it, and then save your changes. This process saves your changes and creates a new version of the document, but it doesn’t protect your version of the document while you are editing it.
Open a read-only version of ...