First, make sure you have the current version of Final Draft 8. In the program, click on the File menu and if you don't have Save as PDF as a menu item, you will need to download and install FD 8.0.3 (Windows) or 220.127.116.11 (Mac OS X 10.12 (Sierra), 10. 11 (El Capitan), 10.10 - Yosemite / 10.9 - Mavericks / 10.8 - Mountain Lion / 10.7 - Lion / 10.6.8 - Snow Leopard or 8.0.1 (Mac OS X 10.6 - 10.4.11). The free installer is HERE.
More information on Final Draft 8 on Sierra is here: I have Final Draft 8 on macOS (Sierra / 10.12) and it crashes when I try to print or use print commands for a PDF.
In FD8.0.3 (W) / 18.104.22.168 (M), there are two ways to do this. If you want to save the entire script as a PDF, go to File > Save As PDF.
A standard Save As dialogue box will come up and allow you to save the PDF wherever you want. There will be a check-box to include or exclude the title page.
If you need only certain pages or scenes or a particular revision set saved as a PDF, go to File > Print (or Control + P on Windows, Command + P on Mac). At the bottom of the Print sheet are options for the export. Check the appropriate radio buttons and checkboxes and when you're ready to export, choose Print to PDF (Windows). A regular Save As dialogue box will come up and allow you to save the PDF wherever you wish.
On Mac, choose PDF > Save As PDF.
A dialogue box will come up. Choose the file name and save-to location and click Save.
At the next window, choose Use PDF:
At the next window, click Print Now to create the PDF. If you want to have the Format Assistant scan your script for errors before the PDF is made, choose Scan Now.
IMPORTANT: Be sure to give the PDF the correct file name.