Unfortunately, if the recipient has the password needed to open the file (the only kind that Word can apply), then with that same password they can open the PDF in Word as an editable file.
To let people read the PDF but not use Word to modify it, you have to apply a password that prevents copying and printing. For that, you need Adobe Acrobat or a similar editor. If you don't have it, you can download Acrobat version 8.0 (an old version) for free -- see http://josephlo.wordpress.com/2013/01/08/acrobat-8-pro-free-giveaway-from-adobe/ for the link.
In Acrobat 8.0, open the PDF you get from Word or elsewhere. Click the Secure button and choose Password Security. In the dialog, choose compatibility with Acrobat 7 and later to get 128-bit AES encryption. Under Permissions, check the box for "Restrict editing and printing..." and enter a password. The "Printing Allowed" and "Changes Allowed" dropdowns should both say None, and the "Enable copying..." box should be unchecked. Click OK and save the PDF.
Now recipients will be able to view the PDF in Adobe Reader or other readers, but they won't be able to modify it (if they honor the security settings, which most but not all do), and they can't open it in Word:
