How make Safari download PDF files instead of opening them in-page?
Open up a terminal window: ⌘space and type Terminal. Hit enter when its highlighted. When you get to the terminal prompt, type the following command:

defaults write com.apple.Safari WebKitOmitPDFSupport -bool YES


You won't get a response at the command line, but once you restart Safari it will no longer open them in the browser.

To take it a step further while you're there, you can keep it from opening them once they are downloaded with this one:

defaults write com.apple.Safari AutoOpenSafeDownloads -bool NO


This last command achieves what the other answer suggests.