Ok, I've tried to manage the process, it works perfectly.
I still have a question.
To get the path and the name of my result file("File To File" parameter), I use two substitution methods :
%x{$FILE_BASE64/tech:path()}x%\%x{$CONTRAT_PDF/tech:physicalName()}x%
Is there a method which allows me to get the full path+name, based on the metadata "CONTRAT_PDF"?