I have been having the same problem on my ipad2 (and iPhone4 as well). I am not sure if it is because of iBooks 2.1 update or iOS 5.1 update. Anyway, the problem extends to ePub files as well. There is no way to get them loaded via gmail, safari, or dropbox. I used to be able to open one PDF file at a time after 3 tries; then I would have to do a hard reset to be able to do another file. Now this method no longer works at all. I have resorted to using stanza, but that is problematic because: 1) it tends to freeze PDF files requiring a reset, and 2) it will no longer be updated. So I am sometimes using kindle app for PDF files. Regardless, none of these temporary measures are satisfying because I can't sync across devices (i am not tech savvy so if there's not an easy way forget it). I don't think it's a capacity issue (<150 pdfs and <75 books) nor a file issue (retried with files that previously worked flawlessly). When i tap open in ibooks from any of those above mentioned sources, the ibooks app launches and either closes and goes to home screen or the pdf or book shows up, flickers and then disappears. The files seem to have a problem "sticking" for lack of better word. For those few seconds the files are visible, they are clear, readable, and seem to be formatted correctly. I hope someone finds a workaround or apple fixes this app.