So your client asks you to do something you are capable of, but which is not your primary field of work. In my experience, this usually happens because it's not too much work, so bringing another employee or 3rd party up to speed (including getting them to be able to work on the documents etc.) is comparatively expensive or otherwise a big bother to them. Basic psychology, I guess.
Let capitalism handle it...
- Make it formal: write down exactly what you are going to do. Separate it cleanly from your main work (graphics?) in your contract.
- Put a price on it that leads to the same (internal) hour-based rate you get for your graphical work. If, doing high quality graphics, you get XX $/h, make it so that you get the same XX $/h for the other work.
This is not primarily so you get paid (I assume that is not a problem), but to make the cost transparent for them.
Probably you will end up with a price that is quite higher than they would pay if they had it done internally, or hand it to some student or whoever is qualified enough to do it (without being qualified for graphics work).
Now, if they still want you to do it, fine! You are not losing anything, really. If all goes well, you will do it for a while, and then they will find a better solution. They will be very happy about you, will be a longterm investment, and maybe even suggest you to further clients. Consider it part of your "relationship work" with that client.
Be sure to be very clear and upfront about this: they want you to do it, you are not the correct one to do it (because you are overqualified), but you will do it for your usual rates if they insist until they find a cheaper solution. They get the person they want and trust; you get the same money as usual. This is both honest, morally acceptable and truthful.
... or just say no.
If, on the other hand, you really, absolutely do not want to do it, but a plain "no" seems not to be the right thing, then how about something along these lines:
"Thanks your your trust in us. We have thought long about it, but the only way we could offer this additional service is to ask our usual rates, the same as for design work. Our calculations show that these prices are much too high for routine Word/template jobs. We cannot, in good conscience, ask too high of a price for this. We suggest that you do these things internally, or find a cheaper way to update the Word documents - maybe a student or part time employee (etc. etc.) can help you better."
If you are not a one-(wo)man-company, then have your boss send that mail; this way your personal day-to-day relationship won't suffer too much, hopefully. The main reason is that it givse you a clear way to say "no" when they come back with it anyways. "I am sorry, please talk to my boss" is simply stronger than "I am sorry, I don't feel like it".