A PDF editor app would be very helpful. The best option would be an app that could edit PDF files like normal text files. But already having the possibility to split or merge several PDF files would be very helpful. Sometimes scanned documents are flipped over, so a rotation function would also be a good point to start with.
My wish would be highlighting pdfs, adding notes. NC could become not only a place for storing pdfs but also for reading and working with pdf documents, scientific papers, for organizing knowledge.