No one has answered this query, which may have not been specific and complete enough in title and description, so let me try again:
When one changes the preferred display colour of the background of pdf pages in "Preferences>Accessibility", this indeed changes the page background colour that is displayed on screen for some files, but not for all. It only does so for files that have text (recognised by Acrobat as such) and no background. It doesn't work for non-OCRed documents, i.e. where pages are seen by Acrobat as images, not text and background (i.e. book pages of non-OCRed scans continue to be displayed as black and white -- or coloured -- images). It also does not work for some (many, in fact) documents that do have text recognised by Acrobat. Many pdfs, indeed, have a background "baked in", so to speak, which is an image/layer. Even with the background display changed in preferences, this would still be displayed as white by Acrobat (just as non-OCRed book pages are generally displayed as white, no matter the accessibility settings). In Acrobat Pro, one can remove this "background image" with "Edit text and images", by clicking the background image and deleting it. This however would be a very tedious task for a book of several hundred pages.
This behaviour of Acrobat is not common to all pdf readers. The small Sumatra PDF program, for example, displays both non-OCRed pages and such pdfs as described above that have a (white?) background baked in with the chosen background display colour when that colour is changed in preferences. Why can Acrobat (a much heavier and feature-rich program) not do this? And if it can, how so?
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