A handy feature in the Preview app is the ability to zoom in on and magnify any area of a PDF document or an image. To do so, press the ` key (it shares a key with the ~ symbol and is typically located just below the Esc key).
Once activated, you can change the area of magnification by moving around your mouse pointer on the document. To increase or decrease the size of the magnification, press the + key or – key, respectively. When you are finished, press the ` key again to toggle off the feature.