Free file locking and more
Following the article What do we want? Where can we find it?: using the Portable Document Format to send files has other benefits beyond locking the words. It will also lock your page layout, which might be useful. Furthermore, unlike some other format, it doesn't preserve your editing. (Although this did generate some good stories, until government press officers realised they were showing their workings and moved to PDF.)
However, you don't need to pay for Adobe software to generate PDFs. Adobe made PDF an open standard some years ago, and it has been incorporated into software including the OpenOffice.org software suite, an open-source alternative to Microsoft Office.
As well as allowing documents to be saved as PDFs, OpenOffice can deal with all Microsoft Office formats and works in a very similar way to Microsoft Office's basic version, incorporating word-processor, spreadsheet and presentation software. It's also free if you download it (www.OpenOffice.org) - although at around 100Mb, depending on the version, it takes quite a long time. Alternatively, it is available for a few pounds on CD-Rom: the web-site has a list of distributors.