If you deal with a lot of PDF documents, you should get Adobe's touch-optimized Reader app. View, search, and comment on PDF documents. You can even annotate and comment on a PDF--add text, draw freehand or add sticky notes with comments. This app ties in with Adobe's EchoSign, to let you sign legally binding documents using your finger.
SketchBook Pro is billed as an artist's app, but that doesn't mean it can't also be effectively used for business. For just $5, this app is pretty full-featured--it even includes multiple layers, brushes, and tools, as well as extra-large canvases. It's great for tablet artists, but it's also useful for creative types in business. SketchBook Pro can be used for anything from creating mind maps or product renderings to taking notes (or doodling) in a boring meeting. The iOS-only Paper app also gets high marks from designers and doodlers. For Windows 8 tablets, check out Fresh Paint.
Handy Scanner Pro/Tiny Scan Pro
Stand-alone scanners are so 20th century. Turn your tablet into a document scanner using apps such as Handy Scanner Pro (pictured) for Android tablets, or Tiny Scan Pro for iOS. These apps let you use your tablet's rear-facing camera to "scan" documents by taking photos of them and applying filters to maximize readability. Then you save the scanned documents as PDF or JPG files.