There are a lot of PDF applications but none of them are quite like LiquidText. This app doesn't follow the usual PDF workflow playbook but instead is engineered around the idea of reviewing long PDF documents better. Using LiquidText you can highlight portions of text and then drag them to a workspace on the right side of the screen. LiquidText collects these excerpts which you can group or comment upon. I like the way I can drag the blurbs of collected text around in the workspace as I try to make sense of them. While it's true there are plenty of applications that let you highlight and comment on a PDF file, LiquidText does it differently.
That, however, is not all. With LiquidText, you can can actually pinch and scrunch pieces of text together in the PDF document. If you have, for instance, bits of text on page one and page four of a PDF that you want to compare, you can just pinch and pull them together. I'd never considered using a gesture to navigate PDFs like this before but it makes so much sense when working on an iPad. As we move to digital workflows, I like that the LiquidText developers are questioning assumption and have a bit of a chip on their shoulder about the superiority of reviewing documents on an iPad over paper.
LiquidText just released version 2. The new version includes an update (accessed through an in-app purchase) that lets you compare multiple PDF documents at once. It's called "Multi-Document" and does a nice job of letting you work with multiple documents at once. You can create a comment in one document that refers to another document at the same time. You can also display documents next to each other and search all of the documents at once.
LiquidText also now supports iCloud Drive, Dropbox, Box, and the other expected cloud document services. You can also import webpages from Safari or LiquidText's own browser.
You can export and share documents from LiquidText with the workspace displayed or not. I've started using some of these features to share reviewed documents with clients and it is working well.