People seem to think there's a magic formula that gets you from a picture of a web page to a web page. But why? There's no magic formula that gets you from a picture of a plate of food to having that plate of food in front of you. You need to know the ingredients and what to do with them (OK, if it's a menu you can get someone else to do the work – well hey, the same is true of web design, you can always either pay someone else or cheat, but either way you haven't cooked your own dinner). The same is true with web design. You don't convert a PDF to a web page any more than you convert a picture of food to a steaming bowl of pasta in front of you.
Yes, a web page has ingredients, and it has a recipe. You need to know what goes into it – the text, the images, and so on – and you also need to know how to put them together – what the semantic context of each aspect is, which bits interact, which bits are fixed and which adapt to the viewport, how the page reacts to different contexts, how hot the oven needs to be and how long to leave it in for and so on.
To go from a PDF to a web page, what you need to know is how to build a web page.
See also a recent discussion along the same lines.