Depends on what you mean by “reducing the dimensions.”
Assuming the image is 300ppi to begin with, If you uncouple the resample box, and tell photoshop to keep the image at 300ppi while reducing the dimensions, Photoshop has to dump some of the data. Under normal circumstances this doesn’t reduce the quality too much. Unless we are talking about a bad image to begin with. For instance any image that’s been repeatedly saved using .jpg compression will have all sorts of crap artifacts in it that only get worse as data is tossed. Sort of the old GIGO rule. (Garbage In, Garbage Out.)
If you keep the resample linked, and reduce the dimensions, you will get a smaller image with a higher ppi resolution, no data lost. But your file size is that much larger too.
BTW, it’s PPI as in Pixels per Inch.
DPI refers only to the print end of things either as a halftone dot resolution for a plate press, or, on an inkjet machine, the actual number of ink dots spewed out per linear inch. With inkjets, it’s totally possible to have two different DPIs, one in the feed direction, one in the head direction. That’s why you will sometimes see 1800x1200 on a desktop inkjet printer (and even the huge production machines are the same way. Step and head can be two different numbers, step usually being the smaller.)