I want to cover a window that measures 40 in by 40 in. If I purchase this product and cut it so it will cover the window will there be seams that will show?

This particular selection is only 12 inches wide, so yes you would have to seam it. If you choose the wider Dusted Crystal (SX-3140) you can use the 48" wide film option and then order 4 feet (based on the size you provided) You would then have approx 8 inches extra in both directions that you would need to trim off. https://www.decorativefilm.com/solyx-sx-3140-dusted-crystal-36-48-60-wide