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Select Zoom Area
on the Preview panel View toolbar.
The cursor changes to
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Select a start point in the Preview panel and drag to an end point in the active view to fence the area to enlarge.
A fence displays as a red dotted line around the selected drawing area.
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Release the mouse.
The Preview panel display updates to show the selected area.
Select Fit View 
on the View toolbar to re-display the entire drawing in the Preview panel.