Microsoft don't make an iPad App called Pages.
To add a password to your document, open the Document inspector, click Document, and select “Require password to open” at the bottom of the inspector. Type the password you want to use in the fields provided, and then click Set Password. A lock icon appears next to the document title to indicate that your document is password protected.
If you want help to create an unusual or strong password, click the button with the key-shaped icon next to the Password field to open the Password Assistant and use it to help you create a password. Select a type of password in the pop-up menu, depending on which password characteristics are most important to you.
A password appears in the Suggestion field; its strength ("stronger” passwords are more difficult to break) is indicated by the length and green color of the Quality bar. If you like the suggested password, copy it and paste it into the Password field.
If you don’t like the suggested password, you can choose a different password from the Suggestion field pop-up menu, increase the password length by dragging the slider, or type your own.