For a device that was criticized at its introduction as being merely a way to consume media, the iPad has been developing targeted productivity capabilities at a rapid clip. Among the latest apps to gain such enhancements are a pair of PDF-reading apps, our long-time favorite from Good.iWare, and  by Readdle. (Thanks to our commenters for pointing out that GoodReader and PDF Expert join Aji's $9.99 , which has had PDF annotation capabilities for some time.)
With Readdle's $4.99 PDF Expert app, you can now perform text highlighting, add notes, and include finger-drawn markup on any PDF. You can also add text underlining and strikethrough. To round things out, the app also includes the capability to add bookmarks to a PDF.
GoodReader (which currently costs $1.99) offers its own spin on PDF markup with a variety of annotation options. The latest version now provides "sticky note" annotations with seven different icons, strikethrough, text insertion and replacement marks, line drawings (including arrows, polygons, ovals, and rectangles), freehand drawings, and highlights. You can also view any annotations that are included in a PDF (and even delete them if you choose) and you can access any files attached to a PDF. Furthermore, GoodReader users can now (at long last) select PDF text and copy it to the clipboard so that they can quote PDF text content in other iPad apps, such as Pages or Mail. For more about GoodReader's features, see "" (5 April 2010).
With either of these apps, your media-consuming iPad has taken yet another step toward being an indispensable business tool when dealing with PDFs that come in for review and comment.