Thanks to henry I now know it was related to the format of the ERB. I have found a way to reuse my html.erb files :
First i do the following in the controller
format.pdf do
render :pdf => "file.pdf", :template => 'contrats/show.html.erb'
end
Then when i use partials i call them like this :
render :partial => 'fullpath/toview.html.erb', :formats => [:html], :locals => { :mylocal=>@something }