One of the most underused features of the iPad is the ability to both read and annotate e-books and PDF documents.  This is a really useful tool for teachers and students who are using their iPad for research.

These three apps allow you to use your iPad to highlight digital documents and hopefully save some unnecessary storage and printing.

iAnnotate PDF

by Branchfire, Inc.

iAnnotate PDF offers complete control over PDFs, but the trade off is learning how to use it. Although the app has a User Guide, it has so many buttons and options that it takes more effort than many of the other apps to use, and much more to master. If you’re willing to put in the effort, iAnnotate is very customizable and will likely fulfill your needs. iAnnotate includes several notable features, including the ability to convert any document to a PDF.

PDFpen for iPad

by SmileOnMyMac, LLC

PDFpen for iPad is the highly-anticipated iPad version of the popular Mac application of the same name. Although the app is new, it has an intuitive UI, a full complement of features, and syncs via iCloud to companion Mac apps. PDFpen offers the most functionality of any app for modifying the original PDF, including the ability to delete or modify its text and images or even create a new PDF. The app is also the only one to implement freehand highlighting in a way that preserves the clarity of the original text, and offers a group of helpful editing ”stamps,” the ability to make new stamps (e.g., a signature), and text expander integration. In fact, PDFpen is so easy to use that it's almost fun. The only negative is that PDFpen occasionally crashes, but the autosave feature ensures no work is lost.

Adobe Reader

by Adobe

Although Adobe Reader is the “official” PDF app, Adobe was late to the AppStore party and only recently updated its app to include annotation capabilities. Despite the delay, Adobe Reader is still a good app, and by far the best free option for PDF annotation. Adobe Reader isn’t limited in any way, and allows users to add “sticky notes” (with the ability to mark yourself as the author), free-hand drawing, and signatures. However, the app has some very significant detriments: (1) it does not allow you to add text outside of a note, (2) no cloud connectivity (users can only import files via iOS’s “open in” function), and (3) no undo or redo (it is possible to delete annotations manually). Despite these minuses, Adobe Reader will likely be enough for many people, or at least a good introduction to the world of PDF annotation.