You need to have Adobe Acrobat installed (the full version, not just the Acrobat Reader).Acrobat appears as a printer so in Stardraw you simply select File | Print, then choose Acrobat (usually listed as PDF Distiller or PDFWriter, depending on your installation preferences) from the list of printers. This will create a PDF file.
What is the problem you're trying to solve by using PDF? If you want to share your design with other people who do not have Stardraw we always recommend using the Stardraw Viewer. This can be freely downloaded from our website, click here to download, and can be used by anyone to view, zoom on and print any Stardraw file.