Use object snaps to draw or edit objects relative to a precise location.
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 for MAC may contain nonstandard snapping points, such as circles with no center points. Use the PDFOSNAP and UOSNAP system variables to turn object snapping on and off.
Object snapping can also be turned on and off by a shortcut menu. Select an underlay and right-click to display the object snap menu option.