Can I modify excel or word doc on the iphone?

Hello,

Is there a way to modify a word or excel document once it has been emailed to my iphone? Sometimes when I go out on a sales call I need to make a small change to the proposal. It would be great to make the change on the iphone, email it to the client right there in his home. He could print it out and we could sign it without altering the document with a pen. In sales it is important to get the signature while the customer is hot!