PDF export as PNG image is of low quality

I had upgraded my iMac with the new macOS Big Sur. All run well until I export a PDF as PNG image, using Preview, which resolution is only 72 pixels/inch. I changed it to 300 pixels/inch and export it as PNG, JPEG, or TIFF, the images are still very low quality.

Then I tried the steps on my old MacBook Pro mid 2012, which was not upgraded to the new OS, it went well. The exported images are of high resolution, and I can choose the resolution freely.

Is it that this problem happens only on the Mac with new OS? Hopefully it will be solved.