Yours Sincerely is short for "I am yours sincerely," and it wouldn't make much sense to say this to someone you don't know well enough to address by name.
Yours Faithfully is short for "I remain your faithful and obedient servant," which is not as emotionally intimate, and thus more appropriate to use when referring to persons with whom you're not familiar.
With e-mail, use enclosed if the document is inside the body of the e-mail message, and use attached if the document is not inside the body - is added as an .