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.
Another potential issue is the set of large buttons on the bottom of the main screen. When you press them, nothing happens. In actuality, they are targets, and you drag files there for printing, emailing and duplicating.
PDF Forms does run well. I imported some PDFs via Dropbox and they were easy to annotate with drawings or typed comments. Adding a signature was easy, and after it was added it could be moved and re-sized. As noted, you can also load any graphic from your camera roll. Overall, the app is very handy and I encountered no bugs. I wish it supported more cloud services directly, and I'd sure like to get the in-app help issues straightened out, but the app itself is excellent.
Other similar apps that are well thought of include PDF Expert 5 ($9.99), iAnnotate PDF ($9.99) and PDF Pro (free) which has a reduced feature set compared to the others.
PDF Forms requires iOS 6 or later and it is optimized for the iPhone 5 screen. I like the app very much -- the user interface just needs to be more intuitive.