Currently I'm aware of two ways of applying a polylength measurement. The standard is to select the tool, and then click along the line. This works very well and having snap to content helps with this process, but it is still involves a number of clicks - especially with a curved shape.
The second method is to apply the polylength measurement to a dynamic fill. This is a really slick way to handle curved measurements, but it has it's limitations. Typically dynamic fill requires a closed area or placement of a dynamic fill boundary to limit the fill area and the polylength is applied around the entire border.
My suggestion would be a tool that creates a limited width fill against a piece of PDF content, and then places a selected polylength along that piece of PDF content. This would be useful for measuring irregular or curved shapes
