Low-density polyethylene sheets can be used for a variety of functions, and it helps to keep a few extra on hand if your company tends to use them a lot. However, because LDPE sheets can be flexible and malleable, you need to store them correctly to avoid damage. The sheets can take a bit of heat and so on, but you want to be sure you're not exposing them to anything that could render them useless in the long term.
Sunlight and UV Treatment
Be sure the storage area is protected from sunlight and UV rays. This includes light coming in through windows; while you can certainly have a window or two in a storage area, the LDPE sheets should be kept out of the light. UV rays can damage LDPE sheets that aren't treated for UV resistance. Even those sheets that do have some resistance should be kept out of direct sunlight just to give them extra protection.
Temperature Range of Storage Area
The temperature range of the storage area is a concern, too, if it isn't climate-controlled. LDPE is known for having fairly low heat resistance; while it can easily take normal room temperatures, and even the temperature on a hot summer day, you don't want the sheets to sit in temperatures starting in the mid-triple digits or so. If you're wondering how on earth your storage room could get that hot, keep in mind that on a hot day, sunlight shining on something can heat up the interior, such as with cars, which can quickly reach extreme temperatures inside even if it feels only mildly hot to you as you stand outside the car. If the storage area doesn't have good heat insulation, the temperature inside the room can climb quickly.
A flat, stiff LDPE sheet looks like it can store nicely in that little space between the wall and a bookcase or filing cabinet, right? It could for a short time, maybe — long-term, not so much. The malleability of the sheet, plus potential heat exposure, could cause it to sag or fold over, making it less usable. You could still heat it up and form it to whatever shape you needed, but doing that to a heat- and gravity-damaged sheet will not only take longer, but it can also be questionable in terms of how strong the finished product will be. A sheet that has bent just a bit may be fine, but one that has been sagging in a corner for months may be weaker around that bent area. Store the sheets flat to avoid all this.
Storage that isn't optimal can have noticeable effects on plastics over the long term. As tough as many plastics are nowadays, you still want to store them as carefully as you can.