Dear all sifu sifu, wana ask something, i'm a fresh graduate and i got offer from company A and they wan me to sign the offer letter, but im still pending for another offer from company B. so right now, what if i sign the offer letter and company B offer me, can i ffk them? what are the consequence? as i havent start working in company A yet.

Not necessary. If you're handing it in person to your superior, usually there's no need for an envelope. Just fold it nicely.
Personally I only use envelope when I'm putting it on my superior's table (in his/ her absence).