I have the universally acclaimed (by Forum members!) iPad app GoodReader. Among its multitude of talents, is the ability to download from FTP servers - no problem at all. It's very fairly priced, frequently updated with new features (always a good sign!) and no one has a bad word to say about it - and you know how willing people are to complain on a Forum!!
Have a look at it in the iPad's native 'App Store'.
Tim
Scotland