About Using Object Snaps With Underlays
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 PDFâ€™s that were created by third
party programs can contain nonstandard snapping points, such as circles with no center
Use the DWFOSNAP, PDFOSNAP, DGNOSNAP, and UOSNAP 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.