Some PDFs consist primarily or even entirely of scanned image files and unfortunately stanning errors occured which result in the images' text being too light and/or background too dark.

For people with great eyesight, this may just be a minor inconvience (if the scanning error isn't too severe), but for users with visual impairments this can easily render PDFs entirely unreadable.

My suggestion is to expand or iterate on the "Night Mode" feature, and add other view modes to assist in better reading, such as a "High Contrast" mode. And if developer/app resources allow, a customizable "Color Replace" mode that can allow users to manually alter recurring colors in PDFs and adjust the text and background colors to a readable state.

Please save the users eyes and state of mind and add these highly necessary visual accessibility features.