Paper degrades if it is near the surface, mixed with loose dirt, and still getting oxygen. If it's packed down under feet of compacted soil and other trash it degrades much more slowly.
It depends entirely on the content of the waste and the levels of moisture and air infiltration.
Some landfills are known to generate huge amounts of methane, and ooze black sludge at the surface. This famously occured after the 1989 San Francisco earthquake.
In contrast, landfills in arid places can preserve trash like a museum:
