Setting DPI Metadata on PNGs that Photoshop can read
Using exiftool cli, run the following command:
exiftool -overwrite_original -PixelUnits=meters -PixelsPerUnitX=ppm -PixelsPerUnitY=ppm inputfile.png
Where PPM is Pixels Per Meters. You can convert from DPI to PPM by multiplying the DPI number by 39.3701