In the notebook section, go to your library. Choose your pattern and there will be a download button. Alternatively, for free patterns, on the main pattern page there will be a link to click to download the pattern or, if it isn't hosted on Ravelry, a link to where you can find the pattern on another website.

Ravelry is a database site. Some patterns listed are hosted on the designer's own website or blog. Some are available through yarn company websites (e.g. Lion Brand). For free patterns hosted on the ravelry site, there is usually a link you can click to instantly download the pattern in PDF format.

If you can purchase through the designer. Ravelry is unscrupulous about providing your purchases

There is a link provided on each pattern page to let you download. As others have mentioned, you can usually save it on your computer as a pdf file, or in your library on your Ravelry account, This is true for the free as well as paid patterns. Occasionally, a link to a free pattern no longer works, or is to a book that you need to purchase. But the paid patterns on a Ravelry always work just fine. Once you buy a pattern, you can download it multiple times, plus keep a copy in your library. It's actually one of the easiest sites I've every worked with.