Open the Underlay Manager. 
It contains a list of underlay styles currently defined for the drawing.
- Click the New button to create a new Underlay Style. The Underlay dialog opens.
- Click Browse button to display File Open dialog and choose a pdf file.
- Now you can see the list of pages of the selected PDF document and a preview picture for the selected page. By default the page name is used as the Name of the Underlay style but you can change it.
- Click OK to close the Underlay dialog. Now you will see the new Underlay Style is added to the Styles List.
- Click OK to close Underlay Manager dialog