I have one for a '69 Camaro and it looks like new. Also have other paperwork from the same dealer.
Last fall I saw an original Gutenberg bible, 558 years old. Probably wasn't always well stored but still looked pretty good. Kept dry and indoors, paper does just fine.
The yellowing comes more from UV light or contact with other oxidizers, between carpet and underlayment is not anywhere close to as archival as a folder out of the sun.
Particularly acids either in the carpet or from rust etc?
I have a build sheet which was so black from the asphalt underlayment it was not legible, I used a 9x13 pyrex and fast lacquer thinner laying it on papertowels and puring the laquer thinner through the sheet, it is nearly back to original coloring and perfectly legible.
