There are lots of reasons why you might want to have a PDF of a score you have created in Sibelius - for the web, to e-mail to students, to include in another document, and many more. Here's how to do it:

With computers running Mac OS X, simply print the file as usual, but select the PDF button at the bottom of the print dialog window, then click "Save as PDF," then name and save the file.

With Windows, you may need a "virtual printer," also known as a PDF creator; there are many free ones available on the web. Install the PDF creator of your choice, print as usual, but select the PDF creator as your printer in the dialog, then name and save the file.