What you want is quite easy to implemement using any number of solutions. (php, coldfucion, standalone software etc) but i think you should think about who the client is a little.
Some of my clients want excel / pdf / doc / printed invoices. I have been informed that "we dont use acrobat" so maybe offer a couple of download methods".
I also find that opening acrobat (instead of right clicking and saving as) can be confusing a little for clients.
I think if you had 3000 clients thne automated would be the corect option but at moment I would stick with aggro free traditional method