Noel,
When using Xara in my work I've had very good success installing and using a pdf creator called PDF995 (it's free with a nag screen).
After you install it and you have your document ready to convert to pdf, instead of exporting, select print then select PDF995 as the printer and you should get a perfect pdf.

As has already been mentioned make sure you actually are drawing within the boundary's of your page size.

Just my 2 cents...

mg