Although there are many solutions offered already, I recommend the following two:
- HTM2PDF - offers an API to convert HTML to PDF and also has a PHP SDK, which makes it very easy to implement in PHP; It offers a choice of server locations in Europe, Asia and the USA
- PDFmyURL - offers an API that does URL and HTML to PDF as well, with roughly the same functionality as HTM2PDF, but works on a load balanced landscape and has been around a little longer
The thing that's different about these two APIs from all the previously mentioned solutions, is that - besides converting HTML to PDF with CSS and JavaScript - it also offers PDF rights management, watermarking and encryption. Therefore it's an all-in-one solution for those who want to hit the ground running.
Disclaimer: I work for Kaiomi, a company that operates both of these websites.