PDF Forms, a universal iOS app, has just about everything you need to edit, annotate, and even sign PDF forms. The US$8.99 app is as full-featured a PDF manager as I have seen, with many features other PDF apps just don't have.
Imported documents can come from iTunes, Google Drive or Dropbox. The app allows you to easily fill out PDF forms, sign legal documents, and add comments via text or even images. When you are done, the app can send those PDFs via email, Dropbox, Google Drive or they can be printed via AirPrint.
PDF Forms is really powerful! I didn't find myself wanting for any particular feature, but in terms of learning what the app can do, it is a bit messy. The help file is a whopping 36 pages long and there is no quick start guide. When I tried to figure out how to add my signature, the on-screen instructions were simply wrong. It suggested I tap a save button that didn't exist. There was a 'done' button which worked correctly, and it is described in the manual.