PDF Forms are not adaptive and neither are HTML5. Adaptive Forms are. There are differences between PDF and HTML5 renderings not from a viewing perspective but from an approach perspective for example to work offline with html5 the AEM forms app would be needed and printing needs a different approach to nam just two. Yes web services can be called from html5 forms. HTML5 can be converted to PDF and a form created out of that result. An XDP form can be rendered as PDF or html5 and used as the form model for an Adaptive Form but it's recommended to use a schema.