Attach a PDF file as an underlay into the current drawing.
Access Methods
Menu: Insert 
PDF Underlay Toolbar: Reference Manager palette Attach ReferenceSummary
When you attach a PDF file as an underlay, you link that referenced file to the current drawing. Any changes to the referenced file are displayed in the current drawing when it is opened or reloaded.
The Select Reference File dialog box (a standard file selection dialog box) is displayed. After the PDF file is attached, you can adjust and clip the underlay from the PDF Underlay visor.