I have been downloading pdf files from my dropbox to ibooks on my ipad, and it has always worked fine for me. Now, all of a sudden, when I open a file in dropbox and tap the arrow on the upper right corner to open it in ibooks, nothing appears: it's just a blank black window. I've tried it with several different pdf files and it's always the same. I can open them in dropbox, but I can no longer transfer them into ibooks.