LiquidText is a superior PDF that allows you to actively review and markup PDFs on your iPad and so much more. The app itself is free. The new Multi-Document pack is on sale for $8.99.
Using the app, you can load a PDF and by pinching, compare pages in different parts of the PDF.
To annotate, you can highlight and select text that can be swiped off the PDF and stored on the side work space. Once the comments and notes are in the work space, you can connect and organize them as you see fit.
By tapping on the comments, you can be directed back to the part of the PDF where the comment is linked to.
This new update includes the ability to purchase the New Multi-Document Pack that lets you work with multiple documents and webpages in a single LiquidText workspace (one time in-app purchase required).
• Pull together excerpts from multiple documents
• Search across all the documents in a LiquidText file
• Compare documents side by side
• Make comments that refer to multiple documents
Once you start using LiquidText, all other PDF document apps will seem like they are missing something. Try the active reading process using LiquidText for a different PDF experience. This app is for the iPad only.