iTunes 11 not allowing to upload anything

Greetings Community. Recently I've tried to upload a PDF file to my iPad/iPhone by drag-and-dropping it to iTunes Library and clicking the required checkbox after that, just as always. But iTunes didn't accept the file - there was only an "unavailable" crossed circle cursor. I've tried to do that with menu File -> Add File to Library, the file appears in iTunes, but when it's uploaded onto iPad/iPhone, it's shown as loading and not progressing at all. The only solution was to change the Media Kind for a PDF to Music and then back to Book, but it's quite inconvenient. Later I've discovered that it's a global problem and same happens when I try to add any music or ePub files with drag-and-drop to my Library. Same thing with PDF happened on my Mac previously, at the moment I'm trying to do this on my Windows PC. I have tried enabling "Manually manage music and videos", but it didn't help. I've been trying to find the solution on the forums, but I have found nothing. The solution is most likely trivial, but I can't find it.