Custom Document fields can be added by clicking on the Plus button in the (Global) Fields manager, and selecting the option Document in the dropdown menu.
The event organizers can edit the name of the field (e.g. Identification document), the label for the field which is visible to contacts and participants (which may or may not be the same as name of the field), and the category the field belongs to (e.g. Personal information).
Any type of document can be added through this field.