If the paper is to be viewed from both sides, an encapsulation mount would be right. And in that case, any kind of reinforcement to the paper should be applied by a conservator.
But if only one side will show, then you could make an Overlay Mount, which is just like encapsulation, except that the back is a 4-ply alphacellulose board instead of a piece of clear film. The fiberous board surface will help to hold the loose pieces of the document in place, so perhaps no direct repairs of the paper would be necessary.
I don't normally have a preference between alphacellulose boards made from wood, and those made from cotton, but cotton boards work better in Overlay Mounts. Cotton boards have a "toothier" surface -- probably due to the fact that the pulp is processed less, and so the fibers are longer.
A 19th century letter, long ago shredded into in four pieces at its folds, is encapsulated and framed in my gallery as a model since 1990. The pieces of the letter are all still snuggled in their correct positions.