Suddenly i can't drag drop pdf files in my iPad 2 !!

hi guys, I'm using iBooks for years and suddenly i can't drag drop pdf files in my iPad2 using the latest iTunes, i searched for a solution which i can sync books but i find it complicated since everytime i want to add pdf file i need to sync my iPad and that's time consuming, others talking about send it by email and then open in iBooks, but what if the file is 20 to 50 MB ? i want it back to drag and drop to my iPad for sure something wrong happend cause it was working perfectly.

please help

thanks