He found-----very interesting to read English stories.

因为只有 it 才能做形式宾语,而 that或this都不能.
你只要记住这个句型就可以了：find / think it + adj.to do sth.[发现/认为做某事……],其中,it做形式宾语,真正宾语是后面的“to do sth”