Until the middle of the 19th Century, paper was made from various plants including mulberry. The best plants had long fibres. Linen and rags were also used to make paper and the Japanese use a whole series of plants with long fibres. The paper made was of high quality and has proved long-lasting. Until then there was no problem about acidity in paper.
Around the middle of the 19th Century it was discovered that paper could be made from wood pulp. This promised to be much cheaper than previous kinds of paper, because you can get a lot of paper from one tree. However, the wood has to be broken down into a pulp before the paper can be made. Chemicals have to be used and these tend to leave a residue of acid in the paper. Of course, there are different qualities of paper made from wood pulp, the cheapest being "newsprint" the paper used for newspapers. Paper made from wood pulp is not so strong as traditionally made papers because it is made from a mush of short fibres and does not have the long strands of (say) linen paper.
But the main trouble with paper containing acid is that it rapidly breaks down owing to the action of the acid content on the fibres, which begin to rot. A newspaper kept in the light will go yellow within a week. Even in the dark it will begin to break down after a number of years. Collectors of comics go to extreme lengths to preserve their comics in plastic bags, changing the bags periodically. The Times used to print a special edition on good quality paper (perhaps it and other newspapers still do) partly for Royalty and other posh people, who could afford to pay more for a nice newspaper, but also so that copies kept in libraries would not deteriorate.
The deterioration does not only affect newspapers. Books made with wood-pulp paper often show a break down and yellowing of the paper. I was once shown round the University Library at Cambridge and they showed me some of the cheaper novels they had, dating from the end of the 19th Century. If a corner of a page was folded over, it didn't crease. It cracked and broke off. Millions of books in this condition exist in libraries and it is a matter of great concern to librarians how to preserve such books (which, despite their apparent triviality, often have important social and historical value.) Methods of impregnation under pressure have been suggested, but they are, of course, costly and the number of books to be treated is in the billions.
About the middle of this century, concerns began to be expressed about the deterioration of books through acidity. The Americans, particularly, were worried and they began to make sure that books were printed on acid-free paper. You will often see this stated on the copyright page of American books. Dover Publications often proudly announce that a book has been printed on acid-free paper.
Of course, more expensive papers always have and are still manufactured, not made from wood pulp and with no harmful chemicals being used in their manufacture. And usually, the more important books have always been printed on good paper. Unfortunately, however, many books which posterity will wish to retain were printed on poor-quality paper before the extent of the problem was realised.
I hope that this information will help your correspondent. I'm sorry for the haste and for the typos, which I'm sure have crept in.
You ask if I know any Web sites dealing with the problem, but I regret that I don't know any. There must be many, but unfortunately I dare not search the web at the moment. Once I get into the Web I am a caught fly and can't get out again.
If you or your correspondent want any further help, just write to me again.
David Lister Grimsby, England.