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My understanding and experience from many years as an archival conservator is that the paper making process switched in the 1980’s from the standard acid process to the alkaline process, primarily on the basis of economics, as it is cheaper to make paper with the alkaline process. The result is that papers made with good quality pulp will be better whereas papers made of less processed, lignin-containing pulp will have an initial alkaline pH but continue to deteriorate. The addition of calcium carbonate to the paper during processing adds weight, and since paper is sold by weight, there is an advantage to the papermaker in adding it though it may just be masking a poor quality paper.
There is an ANSI/NISO standard, the Permanence of Paper for Publications and Documents in Libraries and Archives, originally developed in 1984 and last updated in 2002. Any paper that meets this standard (Z39.48-1992 (R2002) should be acceptable for preservation purposes.
I generally recommend encapsulation for fragile documents only, and store physically-stable documents in acid-free folders and boxes. I don’t like to keep the 3-ring binders as the vinyl covers usually off-gas plasticizers and other undesirable chemical additives. A colleague of mine, a former research chemist for one of the major plastics manufacturers, told me that the plasticizers used to make these kinds of products are often the cheapest (and nastiest) ones available. They are definitely not of preservation quality.
Good luck with your project!