You can add protection to multiple PDF files at once using Actions.
Go to Tools > Action Wizard. Click New Action.
From the dialog box go to Protection > Encrypt
If you don't want Prompts for every file uncheck the option Prompt User & Specify Settings.
Go to Save & Export > Save. Specify Settings
Save the Action and click on the Action from the list on the right
Specify the folder or files on which you would like to apply the Action
Click on Start button.
could you please explain little elaborated.
bcoz i didnt find some options