On the Home screen, tap one of the 5 big cards: Scan with Camera, Sign a PDF, Password Protect, PDF Converter or Extract Text (OCR).
Tap Choose file and select your PDF from the phone or computer.
Choose Add password, type a password (at least 8 characters), and write it down somewhere safe. A lost password cannot be recovered.
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.
After downloading, open the locked file once and try the password, to be sure it works.