Adjust background color for pdf in Preview
I suggest to turn on the night shift function and to push it to a warmer color map: System Preferences > Displays > Night Shift.
This will protect your retina and your brain.
It might be worth trying the Invert colors mode:
System Preferences > Accessibility > Display > Invert colors.
Foxit Reader is an alternative PDF reader, which provides many advantages over Preview, including a Preference setting for the background colour.
(Not affiliated: just a happy user.)