Correction:
WordPad isn't really third party since it is developed by Microsoft, however, anyone could still open a
Microsoft Word Document in WordPad, make changes, and re-save it without ever being asked for the password to
modify.
Another thought came up:
If you were to set the "Password to open" field with higher encryption (i.e. "RC4, Microsoft Enhanced
Cryptographic Provider v1.0") along with setting the "Password to modify" field, then the user would ONLY be
able to open, view, and print the document in Microsoft Word 2002, Microsoft Word 2003, and Microsoft Word
Viewer 2003 if they know the Password to open. Any third party software used to open the file would just error
up and refuse to open the document. In the example of OpenOffice.org 2.0 (and possibly StarOffice 8), it would
say (upon opening the document) that it only supports "Microsoft Office 97/2000 compatible password
encryption".
I still recommend that you send the file as a PDF.