CorelDRAW Graphics Suite X5 :: Vector Objects Behind Transparent Objects Turns Into Bitmaps When Exported

I often produce graphics in CorelDRAW, export it as en eps file and insert it inside a MS Word document as an image and finally convert it to pdf. This way my vector graphics (often linedrawings with or without fill) usually is preserved in the final pdf document. I can zoom in and the lines keep on displaying perfectly sharp. This is important, since it easily looks bad when thin lines are turned into bitmaps, even when watched from a distance.

Now, in some graphics I created today, I had a circle with fountain fill and outline (symbolizing a sphere) put behind a filled rectangle, which had a uniform transparency applied to it. In the resulting pdf file, the 'sphere' was turned into bitmap. Inside CorelDRAW everything was vector graphics, but something was loast in the process. what happens with the graphics in the different steps in the process and eventually could explain a workaround to make everything look as vector in the final pdf file.