Select the drawings to convert to PDF files and then click "Add to batch plot".
The drawings are (temporarily) copied to a branch called "Batch plot". Right click "Batch plot" and select "Choose a plotter".
Select the "DWG To PDF.pc3" option (as shown in the figure below) and click the next three OK buttons.
Result: Your drawings are converted to PDF files and they are in the "Work" folder.
Comment: This option is only available for users who have AutoCAD® 2007 or 2008.