Mac - How to save your poster as a .PDF file

For OS 10.2+ only:
Go to File, Print, choose the options you'd like, then click the "Save as PDF" button at the bottom of the Print dialog.

For OS X (any version):
Go to File, Print, choose the options you'd like, then click on the third pop-up menu to change from "Copies & Pages" to "Output Options". When you get there, check the "Save as File" button, and choose PDF. Now, just press Print.