Is a person considered sinful or an infidel if flies soil the Qur’an with their droppings on the side of its pages, especially if he suffers from obsessive-compulsive disorder regarding matters of disbelief?

The Quran being stained by fly droppings is not considered an act of disbelief (kufr) or a sin, because it is among the things excused due to the difficulty of guarding against them. Al-Iqna' mentioned that fly droppings are excused. Cleaning the Mushaf (copy of the Quran) is better and more impactful in glorifying the sanctity of Allah.