how can I change a pdf filename in ibooks

You can't. The file name either has to be changed before you import it to iBooks, or if it is also in iTunes on the computer you can change it there and sync.

Yes there is a possibility.
From ibooks you can send an e-mail to yourself with the file which has to be namechanged as a attachment.
You can then open this file from your e-mail in an other app.
I am using perfect reader for this.
Open the file in perfect reader and here you are able to change the file name.
From perfect reader you can open it again in ibooks .
Problem solved in an indirect way.
Hmjr