Another PDF question

[masque]

I've attached a file that was created in DesignerPro6 and exported as a PDF/x-1a.
The background colour when opened in Designer is now black as opposed to the red colour behind the house on the rectangle that is due to the shadow area.
Apart form another problem that has now cropped up with this, can anyone suggest a reason why its turned black. (Thats assuming it shows black on any other computer than mine)
In Illustrator or anything adobe its the right colours but I need to know whats going on.
Derek