Only your LORs need to be attested by your college or company. An SOP need not be attested/self-attested until and unless specified by the university. If you take a LOR from your college professor/school teacher or a Principal/Dean, then that LOR needs to be signed by the recommender along with the college/school stamp and letterhead. Similarly, for professional LORs, they need to be signed by their respective recommenders on the company’s letterhead and company stamp.