The main reason is that the way paper was made after the Civil War left a high residual acid content. This leftover acid slowly eats away at the paper, making it brittle, and eventually destroying it. High temperatures speed up this process. And high humidity also speeds the process, and encourages the growth of mold and mildew which will also destroy books and documents. Well, it is possible to neutralize the leftover acid in the paper, but this is expensive and requires some training to do properly.
Most people are better off taking a few simple precautions to protect their books and documents.
Proper storage makes a big difference. Since heat and humidity contribute to acid breakdown, the best place to store books and documents is in your living space. Lamination adds acid and isn't a preferred way to preserve and protect paper. Instead, wrap the items in acid-free paper, available at any art store, or use archival protective sleeves. A professional conservator will be able to help you find these.