Acrobat dark mode that changes the document, not just the app
If you turned on Acrobat's dark mode and the page is still glowing white, nothing is broken. It was never meant to change the document. Drop the PDF below and get one that is actually dark.
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Why the page stays white
Acrobat has two separate settings and they do different jobs. View → Display Theme → Dark Gray restyles Acrobat itself — toolbars, menus, the area around the page. The document keeps its own colours, because as far as Acrobat is concerned those colours are the file's content, not its own to change. Nothing has gone wrong; you have changed the frame around the picture.
The setting that does touch the page, and what it costs
Preferences → Accessibility → Replace Document Colors will force a dark page and light text. It works, but it applies to everything indiscriminately: diagrams lose their meaning, logos come out wrong, and photographs are recoloured along with the text. It also only affects your screen. Email the file to someone else, open it on your tablet, or print it, and it is white again.
Why it does nothing at all to a scan
This is the case that sends people to the forums. A scanned document has no text inside it — the whole page is one image. Replace Document Colors works on text and vector artwork, so on a scan there is nothing for it to recolour and the page stays exactly as white as it was. The usual answer to this on Adobe's own community is that it cannot be done in Reader. That is true of Reader. It is not true of the file.
Converting the file instead
This changes the PDF itself, once. Backgrounds go dark and text goes light, but photographs are recognised and keep their real colours instead of turning into negatives, and charts keep the colours their legend refers to. Scanned pages are handled as documents rather than pictures, with scanner speckle cleaned up first so it does not become bright dust on a dark page. The result is an ordinary PDF, so it opens dark in Acrobat, in Preview, on a phone, and on an e-reader.
What Acrobat keeps
Text stays selectable and searchable, links still work, bookmarks and page numbering are untouched, and the page size is identical. The converter rewrites the colour instructions inside the file rather than flattening pages into images, so Acrobat still treats it as a real document afterwards.
Questions
Why does Acrobat dark mode not change the PDF page?
Because it is a theme for Acrobat, not for your document. Display Theme changes toolbars and menus; the page keeps the colours stored inside the file. Changing the page means changing the file.
What about Replace Document Colors in Accessibility preferences?
That does darken the page, but only on your screen, and it recolours everything including photographs and charts. It also has no effect on a scanned document, because a scan is an image rather than text.
Can Acrobat save a dark version of the file?
No. Acrobat can display a document with replaced colours but will not write those colours into the PDF, which is why the file is white again on any other device. Converting the file is the way to keep it.
Will the converted file still open normally in Acrobat?
Yes. It is an ordinary PDF with different colours. Text stays selectable and searchable, links and bookmarks survive, and you can annotate and print it exactly as before.
Does this work on a scanned PDF, where Acrobat does nothing?
Yes. Scanned pages are treated as documents rather than photographs and get the full dark treatment, with scanner dust removed first so it does not show up as bright specks against the dark background.