It seems a bit strange to want to PDF web pages, but I could see how it would be useful to be able to archive web pages as PDFs. That's what PDFmyURL does, and it has an API.
Used it to create a PDF of an entire blog. It was easier than expected - just had to put in the site's url and later I received a link with a download. If I had to do this by hand it would have taken me days.
quick and easy to use
may not work for every site