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PDF documents allow line, polyline and polygon annotations to contain measurement information to measure the distance, perimeter and area of parts of a document. A scale is defined so that for example, one inch on the document is defined as one foot, allowing you determine the actual size of things in an architectural diagram once you know the scale.
WebViewer has built in tools allowing you to create these annotations and provides a UI to modify common properties. There is also a more advanced programmatic API for fine grained control.
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