The only way to reliably export CorelDRAW compressed drawings (.cdx) to other graphics formats (including .png files) would be to use the original program. This is because CorelDRAW formats are proprietary and usually not supported in other programs.
One thing to remember is that .cdx files are vector graphics and .png is bitmap, so by doing cdx to png conversion you will throw away all the vector data and just get a raster image.