I'm not going to call myself an expert on this (any conservationists here?) But I do work in museums so I have a general understanding of preservation. My perspective is on documents from the 18th century forward.

First, many historical documents were written on parchment or vellum, both of which are animal skin products rather than plant products. Vellum is calf skin specifically.

When we get documents in our museums, often times they've unknowingly been kept in conditions conducive to slowing decomposition.

Archival (written) artifacts decompose when exposed to sunlight and air. Many families keep old documents in boxes or other books that are stored out of sunlight and away from constant air flow. There is a risk of bugs (cursed silverfish) and harsh chemicals in their storage. Which is why archival boxes are acid-free.

I believe, but I could be wrong, that the burying of documents with kings and aristocrats in Egypt in their tombs allowed them to be preserved as well.

In other cases, major historical documents from the time periods I'm discussing were being done by/for libraries that aimed at preserving these records. Such as the library of Congress developing from Jefferson's personal collections. Or the Draper manuscripts going to the Wisconsin Historical Society.

The sad reality is though that many documents do get destroyed over time.

Conservator here with some more information on this!

First, let’s clarify what we’re talking about when we say “on paper.” Paper is a thin sheet of macerated fibers that have been suspended in a liquid environment to be cast as a sheet either by hand (historical process/less prevalent today) or machine (originated in the nineteenth century and by far the most popular way of making paper today). Paper has never been the exclusive substrate for the written word. Bronze age cuneiform tablets were made of clay. More paper-like substrates include parchment, a stretched and dried animal hide product, which has been used essentially since humans first started slaughtering animals and using their hides; and papyrus, layered plant fibers from a reed-like water plant of the same name, which was used in Egypt and other mediterranean areas. These non-paper substrates are subject to the methods of degradation that I’ll discuss momentarily, and plenty of historic examples have been lost and degraded.

I don’t specialize in Eastern papers, so this account is largely from a Western perspective. To explore paper a little more--paper originated in China and spread to Europe around the 11th century. Eastern and western papers are made using different plant products that are local to the region. Eastern papers are still used today particularly in repair because their manufacturing methods and the plants (like kozo) that they use lead to long, strong fibers. That will signal the first big difference in a paper’s longevity--its plant origins and preparation processes.

What we’ve made paper out of in the past several hundred years has changed greatly. Early “rag” papers were made by macerating linen and flax cloth. These plants have a high lignin content, meaning that the cell walls are quite strong and resist deterioration. Papers like this are found in incunable texts, for example, the earliest Western printed (not manuscript aka written by hand) books. Often these old linen papers are in decidedly better shape than more modern papers due to their chemical content.

Even linen papers came in a range of qualities though, as most papermills had rag rooms where women sorted old rags into vats for high, medium, and low quality. (Another aspect of paper making is sizing, or adding an adhesive to make the paper slightly less absorbent to receive ink better, and vats of size could be weak or strong, overcooked, etc. Size quality also changes the chemical content of paper affecting its longevity, but let’s just focus on the major material aspects here.)

As the ability and drive to read increased across Eurpose with the growing popularity of the printing press, the production of paper needed to grow accordingly. More readers need more books, more books need more paper. Papermakers started experimenting with adding other fibers to the paper to make the process cheaper. They added things like straw and other plant fibers, and, fast forwarding a bit, landed on wood as an easy to get material that could produce large amounts of paper.

Unfortunately, what they didn’t know as they made this move to wood, is that wood has a really high acid content. Wood pulp paper breaks down much more quickly because of this, which is why things like old newspapers, made of the cheapest wood pulp paper, yellow and become brittle over relatively short periods of time.

Now, to look at degradation processes. In 1990, Stefan Michalski developed the concept of the “agents of deterioration.” He listed nine agents that would cause damage to heritage items (including books and paper). In 1995, R.R. Waller added a final agent, resulting in the now well known (in conservator circles at least) “Ten Agents of Deterioration.” They are: Physical forces; thieves, vandals, displacers; fire; water; pests; pollutants; light; incorrect temperature; incorrect relative humidity; custodial neglect and dissociation.

Each of these aspects affect paper and other paper-like substrates in different ways and at different rates. Obviously some of these things like thieves (the Isabella Steward Gardner heist), physical forces (an earthquake that swallows up a library), water (Florence flood of 1966), or custodial neglect and dissociation (a librarian misplacing a document, effectively making it useless to researchers) are external factors that affect materials unilaterally. Other aspects relate much more closely to intrinsic aspects of the paper or other writing substrate. For example, parchment is very subject to humidity fluctuations because it is manufactured through drying and stretching, so adding humidity makes it expand and warp. Light can accelerate the breakdown of acids in wood pulp paper and papyrus. Pests may like the proteinaceous size (derived from things like leather scraps or horse hooves) used on a linen paper.

Many materials have been lost to time, unfortunately. Think about the Library of Alexandria and all the information that was lost because of it. But some materials have sat for hundreds of years in environments that, just by luck, happen to offer ideal, stable environments for paper and paper-like substrates--some natural environments are actually better at caring for materials than the most state of the art facilities! For example, the Dead Sea Scrolls, which are a papyrus substrate from as early as the 8th Cent. BCE, were contained in dark, dry, cool caves that preserved them exceedingly well until they were discovered and made available to the public. Unfortunately, huge amounts of damage was done to these inadvertently after their discovery, due to scholars wanting to study them in bright environments.

A lot of effort has been put into understanding and controlling the agents of deterioration since the Dead Sea Scrolls’ discovery, so hopefully something that tragic will never happen again. There are preservation specialists and conservators who work very hard to control environments or counteract qualities intrinsic to paper and other substrates. For example, integrated pest management monitors insect activity, and special collections libraries tend to forbid eating and drinking on site to keep bugs from being attracted to the building in the first place. Conservators may impregnate a particularly acidic page with excess alkaline that “sacrifices” itself to the acids so the paper’s degradation slows. So the heritage field works very hard for materials that we know of to not decompose before we use them, and to avoid damaging newly discovered materials.