Use object snaps to draw or edit objects relative to a precise location.
Underlay object snaps are similar to regular object snaps except that they can be turned on and off separately from regular object snaps, and that they apply only to the objects in the attached file.
Object snapping to PDF underlays is similar to object snapping to drawing geometry. However, object snapping might not behave as expected, depending on how the PDF was created. For example, if the PDF was made from scanned architectural plans, the PDF is a raster image, not a vector-based image. Therefore, object snapping does not work. Also, geometry from PDF’s that were created outside of AutoCAD may contain nonstandard snapping points, such as circles with no center points.
Use the , , , and system variables to turn object snapping on and off.
Object snapping can also be turned on and off from a shortcut menu. Select an underlay and right-click to display the object snap menu option.