I need to convert thousands of Word docs to PDF in a nightly production enviornment

We are having problems with the run time for our daily batch invoices. Currently we are using MS Word and XML to generate the forms. Since they need to convert to PDF at the end we are using MS Word.Interop to print to PDF. We have not been able to get good results printing to PDF using iText or other converters only with Interop. That is the step that is causing the slow down. We are hoping that if we used an Adobe PDF converter that would improve the conversion speed. When I called support they are no help. I think I need a PDF converter that can run on the server, does anyone know if I have to go through a reseller and if so can someone point me in the right direction?  We have hundreds of servers? Do I have to license on each? Thanks in advance.