If the document is a plain PDF (and not a book) you can email it to yourself. Open the PDF in ibooks and tap one so that you see the menu bar. It will have "Library" on the left. Tap onthe third icon in the menu bar and you will have the option to Email the document. Email it to yourself. When you open the PDF in the Email you will have an option to open it in Goodreader.
This does not work with books. These do not provide an Email option.