Inverted colors, dark mode or even night mode. If the name changes from app to app, the feature is the same: changing the page light background color into something dark, and changing its black text into something light. It’s great when reading PDFs in a low light environment and t’s essential when you suffer from some sort of extreme light sensibility, or have a terrible eyesight that makes it so you can’t read dark text on a light background on a screen.
Of all the apps I have tested, those three support some sort of inverted colors, at least partially for the last one.
Here is our reference PDF, for comparison. It is a scan of an old book: a simple black-and-white image, the text in it is not editable/selectable, with some handwritten annotation in red ink.
