Why must every PDF page use the same size and orientation?

PrintSimple prices each job from one finished size and orientation. Mixed sizes, orientations, or page boxes would force someone to choose how to rotate, scale, crop, or impose individual pages. The online builder does not make those choices. Export one PDF with a consistent page setup. You can use PDF/X-4 if your design software supports it. PrintSimple does not require PDF/X, and the browser check does not replace production preflight.