Doc Scan HD is an app that allows you to take a picture of a document with your iPad, and create a PDF file. How this comes in handy for the teacher is that they can take a picture of a worksheet or project directions of which they normally make numerous copies, turn it into a PDF, and finally send it out to the students so that they can digitally complete it on their iPad or iPod.
Great apps for the students to use to write directly on the PDF are: Type on PDF and Paperport Notes. You can find a review of Paperport notes here.
Here is a quick video on how to create your own PDF files using the app: