How do I convert my current HTML and CSS page (with the changes the user did) to a PDF file that can

 Those features which Tim mentioned all use the fact that the user's browser has already rendered the HTML/CSS/JavaScript?/etc page into a displayable (and therefore printable) format.

So if you want to do that on the server side, you're going to need something which simulates that process. I have no doubt that you can find that thing on the web, and I recommend that you do look for it there. Trying to write it yourself would not be a simple process.