Individual companies might have their own payment systems but, basically, you need to pay a fee in order to be given a download link.
You then click on that link to download the file. Depending upon how your browser is configured, it might automatically go to a default location on your computer (usually, and unsurprisingly, your 'Downloads' folder) or you might be asked to choose a location for it.
Once you've downloaded the file, double-click on it to open it in your default pdf viewer (which, hopefully, you've already got on your computer) and then print in the usual way. (e.g. by pressing Ctrl and P together).
NB: With some types of pdf files, and with some browser configurations, you might not get the pdf file downloaded in the way I've just indicated when you click on the link. It might open in your web browser directly, just as a normal web page would. If it does, there will be a 'Download' (or 'Save') icon near to the top of the page, enabling you to save the document on your computer. There will also be a 'Print' icon, enabling you to print it off.