Hi, PDFy owner here. I've been running a number of services for 3 years now, without running into financial problems (see http://cryto.net/, http://cryto.net/~joepie91, http://cryto.net/~joepie91/projectlist). My biggest issue has actually been lack of time, rather than money :)

I've gotten pretty good over the years at running stuff on a shoestring budget (my current hosting expenses are around 100 euro a month for everything together at a large number of hosts, and I have plenty of resources to spare), and as far as I can predict PDFy won't be running into any issues any time soon.

I should add that it definitely helps to custom-develop everything - generic solutions tend to come with a large amount of (resource) overhead, which make it harder to run it on a small budget. By doing just about everything custom, the overhead is minimal. Traffic is dirt cheap nowadays if you know where to look, so that's not really a concern anymore either.