Image resolution explained

The resolution of images in a PDF document is expressed in dpi, “dots per inch”, and determines the amount of information they contain.

The term dpi commonly refers both to:

• The number of pixels per inch in the digital image, which determines its quality. Less pixels per inch (a low dpi value) result in larger pixels, and therefore a poorer image.
• The number of physical dots a printer can place per inch, which determines the printer’s quality.

Images whose resolution is insufficiently high may print badly. The optimal resolution depends on the output device, i.e. for on-screen viewing a low res­olution will suffice, but for professional printing, you need a high resolution.