If the amount of items in your collection are big enough, you may want to really test these paper based product in order to verify their real content of lignin.
Basically, its about to take a clear small paper fibers sample, mix it with a solution and see it trough microscope. Lignin will take an specific colour in contact with the reactive solution.
Its not something hard to do, but as any lab test it should be performed by somebody with previous experience, since you need a positive clear result in order to make desitions. Usually their’re chemists lab operators. This kind of professional with experience in lignin detection and pH mearurement will be not hard to find in universities or labs, so you shouldn’t have any problem finding one.
pH measurement and acidic content is something not hard but complex to understand and we as humans always try to simplify it too much. The fact that manufacturers and simple pH meters are not really regulated makes things harder. Its always worth to demand a professional liquid pH measurement of the materials we are about to use instead to trust the seller or the cheap surface pH meter due to the facts Ron Kley previously exposed.