Before you build it, you might be interested to know that Scribd (and Docstoc, and others) have long had API's that offer this as a free service. We have seen pretty good usage at Scribd, but never enough to think that we could charge for it.

On the other hand, API's for document sharing sites are more complex and offer a lot of other functionality - this may scare off users who just want the transformation. I could see a market opportunity for a Twilio style API that was very targeted at this functionality. I still don't think you'd make a great deal of money charging for it, but I could see it getting some use.