[Ron Wood]

First and foremost, get them sealed up so that air does not get to them. A transparent plastic document protector sealed around the edges with tape will allow you to see the documets but seal out the air. Most early paper was processed using one type of acid or another, so that over time the paper oxidizes and turns darker and brittle. Sealing out the air will slow this process down. Putting them in a picture frame won't help unless you put them between two pieces of glass (which is a good idea) or otherwise seal out the air from the back side of the document. Cardboard backing won't do it. Also protect them from strong light, particularly sunlight, since UV has the same degrading effect.