You have a bigger problem... saving the file generated is the prerogative of the browser. How the browser deals with any particular file stream, even when you set the content type, is entirely up to the browser. Your best bet is probably to use something like ABCpdf to convert the HTML/images into a PDF. I've had pretty good luck with their software, and they have decent support. Of course, this is a third party tool you'll have to install. Without doing that, your next best option is probably to create a zip of the HTML with images and other files (CSS, javascript?)... but that is going to take quite a bit of back-end logic.
Some browsers have this feature built-in. You could ask your users to use that. :)