I saved approx. 50 important PDF-Documents in ibooks-App. Now all of them are disappeared!

I did the last update at 7th of Nov. 2013 before I update my Ipad Mini to IOS 7 (V7.0.3) from IOS 6.

After the update all my PDF-doucments were available in ibooks-App.

Yesterday I would like to check one of the documents and the PDF-Folder in ibooks was empty.

So what happens??

If I restore my IPAD Mini with the last backup (07.11.2013) and I open the ibooks-App again I can see all the doucments, but only for a few seconds.

After that, it looks like they will be deleted and again no objects available any more in the PDF-Folder!!!

Further I did the update to V7.0.4 but nothing changed. After restore - same result!

Could it be, that there's is a bug in IOS7?

If you have any idea please let me know.

The documents are really necessary for me.

Thanks!