Word Opens Documents Read-Only
When opening a Word document, Microsoft Word opens it in read-only mode which precludes editing. Word shows “(Read-Only)” next to the document name.
This may happen if the document was opened by double-clicking the filename in Windows Explorer, and Explorer has the Preview pane enabled.
In this situation, upon clicking the file, Word opens the file to display it in the Preview pane. When double-clicking, Word is launched to open the file but detects that an instance of Word already has the file open. Thus it is opened in read-only mode. Furthermore, the file can't be deleted or renamed because Explorer reports the file is open in Microsoft Word.
This seems to only occur with larger Word files, say, 6Mb or so. Smaller files can be previewed and opened correctly.
To resolve, disable the Preview pane using Organize → Layout and ensure Preview Pane is unticked.
The Details Pane can also cause troubles. See Must Refresh Explorer When Renaming/Deleting Files.