>> It is possible to digitally sign all the leaves of a pdf document at the same time?
No. Digital Signatures apply to the entire PDF and are used to indicate any changes being made to the PDF. You could add a signature stamp or initial stamp to each page and digitally signing the PDF after all the stamps have been applied since stamps are considered a change to the PDF.
[discussion moved from PDF Forms forum]
Thank you GKaiseril. It seemed to me that it is so.