Skim is a PDF reader and note-taker for OS X. It is designed to help you read and annotate scientific papers in PDF, but is also great for viewing any PDF file.
For people who want to use QR Codes within the Flash environment, LOGOSWARE (the company I work for) has released a QR Code Reader into the Spark Project.
It is essentially a library that recognizes and decodes any QR Code from any image source.