One of the most useful tools when preparing CVs and cover letters is Grammarly. This free service does a much more thorough grammar and style check than the basic tools in Word. It will help you avoid “silly” mistakes and make suggestions that may improve the quality of your application.
Another free tool for checking text for mistakes which allows for the uploading and saving of documents and provides real time checking of grammar and also style. Pro Writing Aid has more in depth tools than Grammarly with detailed reports on words used and style.