PDFpen for iPhone is the mobile version of Smile's award-winning PDFpen for Mac. With PDFpen for iPhone, you'll be able to make corrections and changes to PDFs, sign contracts and return them, or fill out an application -- while you're on the go.
I like PDFpen because it provides iCloud and Dropbox support, so you can edit your PDFs seamlessly on your Mac, iPad and iPhone. My friend David Sparks has created a series of screencasts for PDFpen, and the app is available at $4.99 on the App Store for a limited time.
Find out more about PDFpen for iPhone here.