Why in L O Draw a drawing with transparent background exported in pdf loses its transparency

In Draw I created an object on a transparent background. If I export the drawing in PDF the resulting document in no more transparent. I would like to know why and if I make a mistake. My workaround is: from Draw export the file to SVG, import it in Inkscape and from there save as PDF; the transparency is maintained.
Regards.

I’ve created a pdf containing a large range of images. One was a scan & it came out black. I had to change the transparency to get rid of the black background. Damn annoying.