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The Circle tool allows you to draw a circle or ellipse on the document. Hold the Shift key while drawing to create a perfect circle
In ‘Shape', select a square or circle and click and drag while holding down the ‘Shift' key. If you release the ‘Shift' key, the ratio of the square/circle will reflect the current cursor location
Most PDF viewers allow the creation and editing of various annotation types, e.g. text highlights, notes, lines, or shapes, and regardless of the annotation types that are created, PDF viewers conforming to the PDF specification should also support rendering for all annotation types