You can’t use the traditional Save options’s “password to modify”. That will only prevent him from editing the *current* document. He can always do a “Save as…” and create his own version of the document you’ve just sent.
Microsoft offers a costly version of protecting intellectual property though their Digital Restrictions Management (actually the right term is Digital Rights Management), get a digital certificate etc., etc.,
You can do the same thing for free…! no software install required, no additional burden…
Below is the step by step way of doing this:
First open the word document in Word 2007.
Click the Office Button on the top left corner and choose “Word Options”. A dialog like the one shown below pops up…

Ensure that the “Show Developer tab in Ribbon” is enabled.
Close the Word Options dialog and select the developer tab on the Ribbon.
Click the Protect Document Buttion and Choose Restrict Formatting and Editing…

A pane as shown below opens up.
In the second part of the displayed form, Check “Allow only this type of editing in the document” and choose “Filling in forms”
Click on the “Yes, Start Enforcing Protection” in the third part.
Enter a password when prompted. You should remember this password as this is the only way you can modify this document in future.
Now while saving this file, choose “Save Options” from the tools menu in the save dialog box and add a “Password to modify”. I think this is quite easy and it’s also explained in microsoft website which can be found here (http://office.microsoft.com/en-us/help/HA101483331033.aspx#1a)
You are done!
If you open the document now, you will not be edit the contents, copy it, do a select all or whatsoever.
However, there is a catch. A clever person may print this document out to a PDF printer and extract the text by copying and pasting the content from the generated PDF. Most PDF printers obey the Document Security settings applied to a word document. For example, the default PDF printer that you can download as soon as you install Word 2007 will not allow you to copy content from the generated PDF if the main word document does not. However, using some third party tools, users may still be able to extract text. To prevent this, you can write a macro.
Press Alt + F11.
Double click, “This Document” on the left pane to open the code editor.
copy paste the following code.
Sub FilePrint()
MsgBox “Document Printing is protected” ‘ or anyother useful message.
End Sub
Now, when macros are enabled, word will show this message when user tries to print this document. When macros are disabled, word will ask the user to enable macros when user tries to print this document.
To add a double protection to your document, you can also prevent select all, cut, copy paste and other functions in a similar way.
Sub EditCut()
MsgBox “Forbidden”, vbExclamation
End Sub
Sub EditCopy()
MsgBox “Forbidden”, vbExclamation
End Sub
Sub EditPaste()
MsgBox “Forbidden”, vbExclamation
End Sub
Sub EditSelectAll()
MsgBox “Forbidden”, vbExclamation
End Sub
The methods described above works for any office version. Just that the way to access these features varies. I’m using Word 2007, which is why I posted the screenshots in Word 2007.
Hope that helps…
NOTE: such protected files can still be opened with open office and can be copy pasted and printed !! works only with Microsoft Word.