In the meantime, I figured out what was causing the problem.
The PDFCreator context menu works well in Windows File Explorer (I use hungarian Windows, in context menu the Merge item text = "Egyesítés"). When I use the context menu in Windows File Explorer the task is accomplished.
But in Total Commander, in context menu the visible text is "Merge", and I get this message:
"Couldn"t find PDFCreator in your system. Please check if you have installed it."
Could this be fixed, or is it the fault of Total Commander?