False: You cannot create a new clipping boundary without removing the existing clipping boundary. If you want to change the clipping boundary, you have to select the PDF underlay, right-click and choose Remove Clipping Boundary first, and then use the Clip PDF command again with a different option12.

True: To access the Underlay Layers dialog box, you have to select a PDF underlay, right-click and choose PDF Layers2. In the Underlay Layers dialog box, you can click the light bulb icon for the layer name that you want to turn on or off2.

True: To use the Align command, you have to select a polyline, right-click and choose Align3. Then you have to specify two source points on the polyline and two destination points on the PDF underlay that correspond to each other3. You can also specify a third pair of points if needed3. Finally, you have to press Enter twice to confirm the alignment3. The statement "You can use the grips on the polyline to align to the edges and corners of the PDF underlay" is true. When you place a rectangular polyline over a PDF underlay and use it as a clipping boundary, the polyline's grips can be used to adjust its size and position so that it aligns with the edges and corners of the underlay.

By hovering the cursor over a grip, you can see its tooltip display the name of the grip and its function. If you then click and drag the grip, you can move it along its corresponding axis to resize the boundary. This allows you to adjust the clipping boundary to fit exactly over the content you want to display.