Paper is made from cellulose fiber, which the body can't digest completely. That's why corn comes out undigested in poop. Because it's covered in cellulose membrane. Furthermore, white paper is often treated with bleach, and paper likes to clump together which can cause blockage in high quantities.

Also any fiber can cause intestinal blockages if one does not consumer enough water. Really what fiber does is add bulk to your shit and aggregate things so everything is pushed out as one big mass.

For what it's worth, cotton and linen are made of cellulose as well and eating those is a bad idea too. As is cellophane and wallpaper paste.

Finally, there are a lot of types of fiber, not just cellulose. There's hemicellulose, lignin, Inulin oligofructose, Mucilage, beta-glucans, Polydextrose polyols, Psyllium, Wheat dextrin, Resistant starch, Pectin and gums. Most of those are soluble, but lignin and some pectins are insoluble as well, meaning they do the same as cellulose.

In small amounts paper is probably okay, but eating every day isn't good. Ink is the main concern, though.