A lot of us here at TUAW use Readdle's PDF Expert (US$9.99) to view, annotate, and organize Adobe Portable Document Format (PDF) files. The app has always provided a useful and fast way to mark up PDFs, and there's a new update available that adds some very useful features.
PDF Expert 4.0 now supports the Retina display of the new iPad, a feature that works incredibly well. Readdle went through all of the UI elements of PDF Expert and updated them for the higher resolution screen. The result on a new iPad is amazing. The rendering of PDFs has been optimized for the Retina display as well, and text and graphics appear much sharper than in the previous version.
While trying out the updated PDF Expert, I found that the handwriting capture for annotation seemed much smoother on the new iPad. My writing looked less like scrawls on an iPad screen and more like my pen-and-paper writing.
If you like seeing document previews in thumbnails, you'll be happy with that added feature in PDF Expert 4.0. Any documents that you may receive that have embedded video or audio can now be listened to or watched. PDF attachment formats that are now supported by PDF Expert include .mov, .jpg, .num (Numbers spreadsheet), and .mp3.
In the past, PDF Expert did not support PDF Portfolios, which are multiple PDF files assembled into an integrated PDF document. PDF Expert 4.0 adds support for PDF portfolios. The update is available today on the App Store.