One common safe flooring for storage rooms is cement sealed with epoxy paint. Another possibility is cheap light-colored vinyl tile. Cheap tile doesn’t have a lot of plasticizers, which is what makes vinyl off-gas, and light colors means there’s a lot of calcium carbonate, which buffers any acidity. Plywood – even marine plywood, which off-gases less than regular – shouldn’t be used in storage. Storerooms are often built with little ventilation, which makes them easier and cheaper to keep at a steady RH. So anything that off-gasses can build up in the space. Sealing plywood doesn’t work over the long run; sooner or later the coating will start to crack and then all the fumes that have built up will start to come out.