Make a PDF background black
For a page background that is properly black, not dark grey, this is the style built for it.
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The OLED style
Among the four available styles, one is called OLED and its background is pure black — #000000, not a dark charcoal. It is built for OLED and AMOLED screens, where a true-black background means those pixels turn off entirely, which is both the deepest possible black on screen and lighter on battery over a long reading session.
Text on true black
Body text on the OLED style becomes a light grey rather than pure white, because pure white on pure black is harsher to read for long stretches than most people expect. The result still comfortably clears the contrast needed for body text, it just isn't glaring.
Everything else still holds
Making the background black does not mean everything on the page turns black and white. Photographs keep their colours and are dimmed rather than inverted, charts keep their data colours, and text stays selectable — the background is the only thing pushed all the way to black.
Questions
How do I get a pure black background, not dark grey?
Choose the OLED style in the converter — its background is pure black (#000000), as opposed to the default charcoal style.
Is black background better than dark grey?
On an OLED or AMOLED screen, pure black uses less power because those pixels switch off. On an LCD screen there is no power difference, and some readers find a dark charcoal background more comfortable than pure black for long sessions.
Will photos and diagrams still be visible on a black background?
Yes. Photographs and charts are classified and preserved rather than inverted, so they still look like themselves against the black page.
Does a black background affect printing or file size?
The file stays close to its original size because the colour instructions are rewritten rather than the pages being turned into images. Printing a black background will use more ink than a white one, so this is meant for reading on screen.