If you want the resulting PDF to capture the look & layout of the web page, then I think you're talking about driving the browser, i.e. with Selenium. It's easy to drive the browser to go to certain sites, look for elements on the page, and so on; but I'm not sure how easy it is (or if it's even possible) to access browser features such as Print. Assuming it's possible, then probably what you'd want to do is configure the PDF virtual printer to be the default on your system, with the 50% scaling, default save location, and all that. Even then, there may be some GUI elements you have to access manually, so I'm not sure that this is fully automatable.
An alternative approach might be to use a web service out there somewhere which converts web pages to PDFs. For example, there's PDFcrowd. However, with this one, it looks like the options you get for free don't meet your needs (scaling, for example). You could search around for others.
Updated to fix some tpyos.
I reckon we are the only monastery ever to have a dungeon stuffed with 16,000 zombies
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