To save your lecture notes as a PDF file for offline use, choose the print icon in the Notebook toolbar and then the Print button in the print window.
If you have a version of Adobe Acrobat installed on your computer you’ll be able to print with a PDF printer. Choose Adobe PDF as the printer.
If you do not have Adobe Acrobat on your computer, there are plenty of free PDF printers you may download from the Internet. Check out this site: http://helpdeskgeek.com/free-tools-review/7-free-pdf-writers-and-pdf-printers/
If your computer is a MAC, you can also choose save as a PDF from the PDF pop-up menu.