This .pdf parafoil kite plan is from https://www.kiteplans.org/planos/parafoil4/parafoil4.html, but I have used Google Translate. As you can see, it is about 35 years old, but I liked the approach by the author. It isn’t a plan to tell you how to make it – it just concentrates on quality calculations to create the patterns.
I didn’t use this so much as a ‘plan’ as for the shape of the aerofoil and fins. I relied on an element of trust when it came to some of the maths…