set TargetFolder to choose folder with prompt "Please choose the folder where to save the PDFs"
tell application "QuarkXPress"
    set MyProject to object reference of project 1
    tell MyProject
        repeat with i from 1 to count of layout space
            try
                export layout space i as "PDF" in ((TargetFolder as string) & name of layout space i & ".pdf")
            on error errMsg number errNum
            end try
        end repeat
    end tell
end tell