Probably some web style leaking into your PDF page design.
Put a button somewhere to open the PDF page directly, then you can inspect the grey area to see where the grey background comes from. I always have some admin page or admin button to open the PDF generation pages directly, to inspect styling and check data without waiting for the PDF to be generated and opening it.
Check out my blog to find more details on the classes you need to use in your styling.
https://medium.com/mendix/pdf-document-generation-in-mendix-a95e67d41503