No, not plastic folders, unless the "plastic" is actually acid-free archival-grade polyester. You should not mount them, either, unless you have a place in your home to display them where they will not be exposed to UV light (and if you do mount them, be sure that the mount is an acid-free dry mount under UV glass.)
I would suggest ordering a document box and some polyester envelopes for them. Either Light Impressions (lightimpressionsdirect.com) or Family Archives (familyarchives.com) will sell these in small quantity. You should be able to get a box and a small package of envelopes for no more than about $50.
PS: The spray that Bookgirl mentioned is called Bookkeeper, Light Impressions carries it. It's very expensive, however, and probably not necessary unless the documents are already becoming brittle.