Now I give up...after many hours of searching for the answer how I save my vector logotype text with a transparent background as pdf. I can export the file as an png and my white font appears on an transparent background but I need it in vector so that I can enlarge the picture with the same quality. Hope anyone can help me....

It should work with PDF too, but when you open a PDF file it renders transparency over white background so your white font is invisible. Import the PDF back to Inkscape, change the background color or create an object behind everything to check it was exported properly.