How to print transparent images in Win32 (color key)

I am trying to print a transparent image in a Win32 application . The image has an alpha channel, but I can live with a simple transparent mask too.

When using the :: AlphaBlend method, it displays correctly on screen and prints correctly on my HP ColorSphere printer. However, with PDF printers (like Primo PDF), the image becomes opaque in PDF.

When setting the same image in Word, when the transparency color is white, the image will appear transparent when using Primo PDF.

How do I draw an image so that transparency also works in PDF printers? Or: how does Microsoft Word achieve this?

Thank,

Fabian