Paper produced before about 1850 was generally made from
bleached cotton rags, a
process which results in
acid-free paper.
Paper produced since 1850 is almost always
wood-based (i.e. made from
wood pulp).
Wood-based paper has a high
acidity and, consequently, deteriorates fairly quickly.
This truly epochal change in the paper manufacturing process explains why books and other paper documents produced prior to 1850 are often still in better condition than those published shortly after 1850.