plot in pdf with images with transparency

I try to make a PDF with some transparent images. With the new PDF-Export I get a very big file (500mb) and the background of all transparent images is black (should be white). The same happens with plotting the layout with adobe acrobat or ghostscript (but the files are small, 10 mb). With Autocad the plot of the same file with ghostscript and adobe acrobat works fine (background of transparent images is white).After turning transparency in Bricscad off for all images the export as PDF (all ways) works, the background of all images is white (as it should be). The problem is I need transparent images in the plan.Is this a bug?Kai Otte-Witte