To turn on anti-aliasing:

Choose Preview > Preferences, and click PDF.

Select the “Anti-alias text and line art” checkbox.
When you view a PDF document in Preview, larger text may look jagged. You can turn on anti-aliasing in Preview to make the text more readable.
Choose Preview > Preferences, and click PDF.
Select the “Anti-alias text and line art” checkbox.
If you turn on the “Anti-alias text and line art” option, large characters and line art are smoothed out and do not appear as jagged. Preview may take longer to display your document when smoothing out characters and line art.