הלכה אויף אסתר 9:29

All of a Scroll needs to be sewn to the point that all of its hides be one scroll. And it can only be sewn with sinews, like a Torah [scroll]. So if one sewed it without sinews, it is disqualified. But one is not required to sew all of its pages with sinews, like a Torah [scroll]. Rather even if he sewed three stitches with sinews at the end of the page, three in the middle and three at the other end, it is fit — since it is called a letter (iggeret).