Adobe Air (Adobe Integrated Runtime) is the runtime environment for building and running applications using using Adobe Flash, Flex, HTML and Ajax. It's more accurate to say Adobe Air is the mobile version of Flash (although there actually is a mobile version of Flash) as Air uses the Adobe Flash Player as its own runtime environment. There are
lots of apps (like WatchESPN) which require the Adobe Air runtime to be installed. Adobe Air allows you to run Web applications as native apps outside of the browser.
One of the best readers for PDF files is, oddly enough,
Adobe Reader. The various office suite apps also read PDFs. One better than Adobe's own reader (which is free) is
ezPDF Reader (which is $2.99). Another good one is
RepliGo Reader.