The print command prints a layout, it has nothing to do with PDFs or container fields, unless you've rendered the contents of that PDF to your layout. It's easier to just launch the PDF and let the user use the system dialog to select number of copies.
There are printer plugins for filemaker that allow you to have granular control over printing if you need it:
I understand your point about managing te pdf outside FM.
How could i launch a pdf from a dropdown menu to begin the process ?
Use a script trigger to run a script that exports the container field contents from a specific record based on the dropdown value. OnObjectModify is probably what you’re looking for.