It's important that you cool the pulled pork relatively quickly. When pulling, spread on or in sheetpans or roasting pand (you can chill the pans first to facilitate cooling). Do not pack warm or hot pork into bags or pans and then fridge. It will not cool thoroughly in the fridge if packed thickly and the growth of both spoilage and pathogenic bacteria becomes quite possible. Spread the meat out on the pans, rotate into the fridge to cool well, pack only when cool (<70F), and fridge.
If you want to pull later, separate the muscles and cool on racks till the edge is taken off, then (on racks still) in the fridge till cold. Reheat as Ron suggests.
To reheat in a pan you can moisten the meat, if necessary, with stock, juice, sauce or a combination, then place n a 300 oven, covered with a lid or foil. It is not necessary to reheat slowly but it is necessary not to cook the pork again. Leave it alone for 20-30 min in the oven then stir, replacing the cover, then stir every 7-10 min thereafter, till the pork is at serving temp throughout--just hot. Serve immediately or, if you need to hold it longer, turn the oven down to its lowest setting (or off) and keep covered.
If reheating whole pieces it is only necessary to take them to a bit past serving temp (145-155)--to 155-160 or so, no higher as then they start cooking again.