The problem you are facing is totally irrelevant to Mailman. You need to change your PDF printer settings.
Printing PDFs can be frustrating based on what virtual printer you are using.
But most of PDF printers have got some settings in the line of “prompt for file name” which you can disable. Then your PDFs will be printed and saved without the save dialog popping up.
Some of them might be trickier to set and involves changing the registry key. Have a look at this: http://archi-lab.net/printing-pdfs-from-revit-why-is-it-so-hard/ if you want to do that programmatically. However that can be done using Python as well as opposed to C# which is shown in the link. Another thing worth being aware of is that PDFs can take time to be saved. That means the file in the given path might not be available for Mailman nodes when it is looking for it. You can use this Python code to delay this process until the file becomes available:
sys.path.append(r'C:\Program Files (x86)\IronPython 2.7\Lib')
from os import path
from System.Threading import Thread
pdfFilePath = IN
while not path.exists(pdfFilePath):
OUT = pdfFilePath