If you have a document, such as a passport or marriage certificate, and you don't want to supply the original, then you go to a notary or a solicitor, get them to look at the original and take a copy or look at the one you have provided, and then certify or notarise the copy as a true copy.
Things like downloads of statements, bills etc can't be notorised or certified, as there is nothing 'original' to certify them as copies of.
Vicky