I have four different ebook apps on my iPad (iBooks, Kindle, Nook and Marvin). None of them gives the possibility to send files to Dropbox directly. With Kindle and iBooks, I can send PDF files to one of my email addresses per email, then open it in one of the apps. See my answer to your question in your other thread.Is there an app that can read ePubs and PDFs and has the facility to output files to Dropbox to send them to a PC?