Brightness/Exposure on photos airdropped from my iPhone is off off on Preview

Do you have HDR turned on? If so, have you tried with it turned off?

It may be that Preview is trying to display this picture as color or a grayscale that would make pictures on the page look best rather than text. I tried it with a monochrome page with old photographs and text, and the text looks better if I use the menu "Tools>Adjust Color" to increase the Exposure and Contrast. (But the photographs look worse!) I set them about halfway between normal and max. My picture had wrinkles in the page, so it also helped when I increased Shadows compensation..

Does that work for you?

The images I'm using to test with all open the same in every image viewer, so Preview is doing what it's supposed to do with these images. You could try converting them to PDF by right clicking and choosing Quick Actions. If it works for you, you can do this with multiple files at a time.

If you're taking the pictures, you'd probably be better off using the Notes App and choosing (•••) More>Scan.