Since anyone reading this far must be into plastic let's add that the soft vinyls are not only bad for documents, but they can melt when in contact with styrofoam. Here's something I wrote in a 2013 thread about this. This concerned a foam head metling onto the synthetic vinyl sweatband inside an overseas cap.
The culprit is the PVC. The plasticizers leaching out attack the styrofoam. Remember the "new car" smell in those big old Detroit cars of yesteryear? That was the smell of the nasty chemicals in the seat covers, etc. outgassing into the air. If you leave electronics gear in the original foam packing for a long time, you may find that the PVC power cord has foam attached to it from the same leaching process.
Leather would not do the same thing to styrofoam.
There is a different problem with the kind of soft, usually grey, foam used in some small plastic medals cases (it's also used in earphone pads, flight helmet pads, etc). That stuff breaks down over the years and gets hard and crumbles to dust if you touch it.