I'm looking for an app that can help me keep several hundred PDF files organized. I want to group the files into folders, and within each folder, arrange the PDF files into a reading list in any order I want (like an iTunes playlist). Ideally, I would be able to read and annotate the files within the app (iBooks can keep files organized the way I want, but doesn't have PDF annotation/mark-up tools). Does anyone know of an app that can do this? Thanks.