You ideally want archival polyester sleeves with UV protection. Usually these are mylar, but not all mylar is archival.

Non-archival plastics can be coated, will break-down over time into mildly corrosive compounds, and don't offer UV protection.

Then best practice is to keep those documents in an acid-free file folder, and then keep those folders in an acid-free archival box.

And if these documents are stored vertically rather than horizontally, you want to make sure that they can't 'sag' in the box. Otherwise they'll acquire a curve that doesn't readily come out. Archival document support spacers are the way to keep things fully upright.