It depends on what you mean by "save it as pdf".
If you mean to save the tree as you see it on the screen but to a pdf file, then click the "Print" link at the bottom, then in the sidebar, click the "Download PDF" button. You'll be given an option to Open the PDF file (e.g. with Adobe Reader) or to Save File (your browser determines where it will be saved).
If you mean to save all the information you've entered to a PDF file, then click on the "Download / export this family..." button at the bottom left. Though PDF is not an option, you can save it as Plain Text.
