Transparent RTF Text over watermark

When using wpTools with wPdf I took a PDF-File for the background as a watermark. But the RTF File has a white background and this overlappes with the watermark and makes it partially invisible.

Is it possible to put out the RTF file with a transparent background ?

I'm using WPPDFExport - where can I find the wPDF.DeviceOptions ?

When I set

WPPDFExport1.Modes := [wpWhiteBrushIsTransparent];

it's the same effect, as before.

All paragraphs color and shading is set to 0.

I think, the problem could not be solved with this.

Do you have anothe idea ?