Excel is dependent of the default printer when it converts files to PDF. PixEdit® Converter Server always uses A4 as paper size on conversions where Excel is used. But, if the default printer is configured wrong it could lead to a different paper size, or it may not be able to convert anything at all.
To ensure that the default printer has the right settings, log into the server with the same user that is running the PixEdit services, open "Devices and printers" and always select "Microsoft XPS Document Writer" as the default printer. If you want the paper size on the converted documents to be A4, also check that the default paper size for the printer is set to A4.