>Any recommandation for a simple efficient converter from docx to pdf?

I would recommend the automatic solution by Christian Isberner. You can use that code on a server. If you don't want to write the automatic code in C#, you can create a simple VFP exe that you run from C# and it will externally process the file and return to you when it's completed.

It's the only way to make sure all valid .docx files are handled. It will follow Windows natural updates on installed tools, etc.