Fill PDF Service is an API for filling in editable PDFs with data from a website. It is most commonly used with the Fill PDF Drupal module, and that is what this site uses to provide the demo (along with the service itself). That said, Fill PDF doesn't require the service, but many people prefer it since using it standalone requires setting up Tomcat and a custom servlet or installing pdftk on the Web server. Fill PDF Service just requires an API key.
Try the demo or learn more about Fill PDF Service.
If you are unfamiliar with Drupal, check out our Learn Drupal page.