A similar service is currently selling on flippa: https://flippa.com/2793470-htm2pdf-co-uk Maybe you can steal some ideas from them. Considering they're PR6 and seeing only low 3 digit monthly revenue, you probably shouldn't expect great wealth to come of it.
To be honest, I don't really understand the benefit over just hitting ctrl-p.
I may be mistaken, but printing a document to PDF is not something one can do with Windows. At least not without buying Adobe, or installing Ghost.
Unless things have changed with Windows 7?
I thought it would be a big feature to let users transform many urls in one step. That should be an improvement over printing all the urls separately. Perhaps as a premium service?
Having said that, dont let it stop you because your site might be useful for the layperson who doesnt understand all this stuff about installing something as a fake printer and then printing through that.