When you select a PDF underlay, the PDF Contextual tab is displayed on the ribbon.

Adjust Panel

Contrast

Controls the contrast and the fading effect of the underlay.

Fade

Controls the appearance of the linework. The greater the value, the lighter the linework
in the underlay is displayed.

Display in Monochrome

Displays the underlay in black and white.

Clipping Panel

Create Clipping Boundary

Deletes the old clipping boundary (if one exists) and create a new one.

Remove Clipping

Deletes the clipping boundary.

Options Panel

Show Underlay

Hides or displays the underlay.

Enable Snap

Determines whether object snapping is active for geometry in the underlay.

External References (EXTERNALREFERENCES)

Opens the External References palette.

PDF Layers Panel

Edit Layers (ULAYERS)

Controls the display of layers in a DWF, DWFx, PDF, or DGN underlay.