Is there any way to do this in Drupal 7?
I want new users send some documents via Webform on registration.
The reason is that I don't want to save these documents as an user profile field.
It sounds like you want the Webform Registration module. It lets any webform create users upon submission. It also lets you customize the registration process by selecting which webform elements are associated with the newly created user's name, password, & email.