Convert PDF to black background with white text within Apple Books?

I have eye issues and can read in the Kindle app pretty well since I can convert the background and text to black/white accordingly and increase the font. This is true when importing PDF format. Unfortunately the Kindle app won’t let me import files larger than 50mb so I needed an alternative. Apple Books easily imports larger PDF files but only in the original format (white/black) and it makes it tough on my eyes. I’ve done some of the suggestions offered in older posts with no luck. Anyone have any ideas on this subject? I’ve tried online conversion but after claiming no limits they all failed and wanted me to purchase the software. Go figure.