At a time of receiving digitally signed document it is decrypted with sender's public key. But how receiver knows that public key?? Does it(public key) go along with digitally signed document?? If so then how to extract it at destination??
You don't mention in which context you're concerned about this, but generally the key is not sent automatically along with the document. The receiver needs to get hold of it by some other means. Many people keep the public key on their web site, so if you get an encrypted email from them, that might be where you get the key.