Forgotten password

If you don't use the Remember this password in my keychain option when setting a password, then you need to keep document passwords in a text file that is both stored on your Mac, and updated, printed and stored in a safe place.

When you do use that remember option from the dialog above, Pages will ask the Keychain to automatically supply the document password when opening the document in your account, and there will be no password prompt. Outside of your account, the document will present a password challenge.

Unless you have a good memory, or can refer to a saved list of passwords, there is no resetting your forgotten Pages password.