That's the way Word works by default. Word documents can be shared by email and edited by anyone.
You can password protect your document. On Windows, open the document, go to Tools, then click Options and then Security. On a Mac, open the document, go to Tools, then Protect Document...
I've never done this myself, but people at work do it all the time.
I circulate Word documents as pdf files if I don't want anyone to change them. I just save as pdf in Word 2011 on my Mac. I don't know how to do it on a vanilla Windows PC - sorry.
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