Although other apps have offered the ability to annotate or mark up PDFs, Easy Annotate is one of the few, if only, app that allows side by side editing of PDFs.
You can learn more about the app on the Easy Annotate web site and watch the video below.
The developer describes the app’s features as follows:
– Side-by-side Editing
– Linking PDFs
– Full Dropbox support
– Annotate (highlight, underline, cross-out, draw)
– Print, mail, export
– Easy to use interface
So, if you are interested in having the ability to view and annotate PDFs side by side, you should check out this app.