Open Print-ready from the Home screen (or use its own page link).
Tap Choose file and select your PDF from the phone or computer.
Choose the Paper size (A4, Letter, Legal or A3) that your printer uses.
Choose Double-sided.
Tap the big purple Start button and wait a few seconds.
When you see Done!, tap Download. The new file is saved on your phone or computer.
For double-sided printing: print the first file, put the printed sheets back in the tray as your printer manual says, then print the second file on the other side. Test with one sheet first.