A family of Microsoft word processing software products for creating web, email, and print documents.
One cause of this problem can be the Preview Pane in Windows Explorer.
If you try to open a Word file via Windows Explorer and you have the Preview Pane open the first thing that will happen is that the preview window will start reading the file to display the preview of that file, in the meantime Word itself will also start reading the file. Sometimes the two process will conflict and Word will determine that it can't open the file for editing because the file is locked (by the preview window) and therefore Word will open the document as read-only. Large files and files with certain types of content can cause the file to be locked by the preview pane.
If this is the scenario that is causing your problem either disable the Preview Pane in Windows Explorer or open the file from within Word via File|Open.