I is indeed non formal, and some people may like that , but to make it a high quality document that is to be considered professional.... "I" needs to go . As far as I am aware absolutely no rpg books say "I" when explaining the rules (the obvious exception is when explaining in character game play). You dont HAVE to do this, but in my opinion this PDF is high enough quality for selling , but to do that you must not use "I" .
In my zelda RPG I have sections called game design theory where I talk informally about the rules I chose, however my final document will not have this. Except maybe in the appendix. You could do something like that I suppose. However, rules are no place for first person point of view. I'm honestly just trying to help make this a professional document.