Apply company paper on excel sheet

I designed our company formal paper on illustrator and i want to apply it on excel sheet as built in so everytime i use save as PDF its already on the company paper


I tried but i never get the sizes correctly and it always looks wrong

IMPORTANT NOTE
i have water mark on the company paper (Big S in tbe back) must be in the excel sheet as well!

I have attached PDF AND AI AND EXCEL SHEET FOR REFERENCE