They must? Or else what? And whose legal system is going to force them?

You simply cannot guarantee that the recipient will see what you want them
to see, because the user may have set their e-mail client so that it only
sees plain text, for example. Many people have no choice in the matter
because of corporate firewalls/policies. They do not nececessarily have a
PDF viewer, either.

Word's merge to e-mail certainly won't give you very much control over this
even if you use HTML format - it's not designed to do that as far as I
know. - i.e. if you are struggling to achieve what you want via Word, you
would probably be better off looking for a 3rd party commercial package that
is geared towards mass HTML or PDF format e-mails.

Peter Jamieson