DS renders do nor have a PPI value assigned - your editor/viewer will probably assign one, either 96 or 72 according to the software/preferences. Remember that PPI is just a number at the front of the file - an render 1,000 pixels square at 300PPI is exactly the same as an image 1,000 pixels square at 96PPI. If you need an image of a set physical size at a set PPI just type the numbers as a sum - e.g. for a 6" by 4" image at 300PPI type 6 * 300 and 4 * 300 into the size fields.