Have saved about 100 hymns that I use for church. I save them as PDF files and add new entries every week by using Dropbox. Each hymn is about a meg. Today I added 2 new hymns by opening them in Dropbox and then moving them over to iBooks. The 3rd hymn wouldn't save as an iBook pdf document. I see the file for a second in iBooks and then it disappears. Have I reached a limit of some sort?