I am not aware of any iPad app that will do what you want.
Even on a computer apps to convert PDF's back to editable documents are few and quirky in execution. When you output a document as a PDF you lose a lot of the information from the original document. Putting it back in is mostly a matter of guesswork, and most apps are poor guessers. Easy enough to get the text, if it's not all graphics (which it can be). Almost impossible to get the layout back as anything like the original.