(1) The main reason to use a PDF is that it is far harder (not theoretically impossible, but not demonstrated in the wild yet, either) to embed a virus in a PDF document than in a Word document. RTF is fairly "secure" in that sense, too, but is more prone to problems with character-set mistranslation between Macs and PCs.
(2) Jim is almost correct, and for the purposes of anybody on this board gives sound advice. Although there are a few really strange circumstances in which registering one's copyright in advance of submission is a good idea, those circumstances are quite rare—and if you're in them, and you don't have assistance from a good literary attorney, you're going to be in far more danger from other legal issues than from failing to register the copyright before submission! In other words, if your (competent, experienced) lawyer who practices publishing law says "I think you should register this manuscript before submitting it," do it; otherwise, save your $30 for the nonce.