As I said in my review of PDF Expert 5 for iPad:
"The five points of focus in this update are the new, clean, flat and iOS 7 style redesign, a completely revamped file manager with a bunch of new features, a new PDF Viewer with a full screen mode, a Text To Speech mode which makes use of iOS 7's text to speech API, and a brand new and revolutionary Review Mode."
If you just moved from PDF Expert 4 to version 5 on the iPhone, you'll basically have the same experience. You'll be guided through a migration assistant which retrieves your important files from version 4, and you'll instantly appreciate the new design, the much better file manager with improved drag and drop support, new features like annotating, editing, filling in and signing form, and a faster overall experience. What's also really nice is that your iCloud files automatically are available in the new version as well.
This update also has three new features including continuos scrolling, calculations in LiveCycle Designer Forms, and more speed. Continuos scrolling works as you'd expect it to and comes in really handy on the iPhone version. It lets you read much fast and have a better overview of the document you're editing. It's blazing fast even with big documents and searching the document happens instantly.
Unfortunately you don't have the great inter-app communication as you have on the iPad, but with this app going universal, maybe we'll have more possibilities in the future. Nevertheless PDF Expert 5.1 works perfectly fine on the iPhone and keeps getting better with each update.
If you already bought PDF Expert 5 for the iPad, this is a free update for you, and if you're new to PDF Expert, you can get it on the App Store for $9.99.