SharePoint only protect your content by controlling who could access the documents, however, it will not be able to protect the documents outside its environment. This is where you would need Windows Rights Management Services (RMS).
This is a great walkthrough published by Russell Wright from SharePoint Solutions if you are looking at protecting your SharePoint content using Windows Rights Management Services (RMS).
http://sharepointsolutions.blogspot.com/2008/01/single-server-rms-and-moss-2007_06.html
For more information on Windows Rights Management Services, here is the link to the product home page:
http://www.microsoft.com/windowsserver2003/technologies/rightsmgmt/default.mspx
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