Can extension (content script) run in the default PDF viewer?

[Juraj M.]

I always thought the answer is no, but I see the Dark Reader actually works there.

But how? Is it just for CSS or is there some special way to inject it?

[wOxxOm]

When using the default PDF reader the tab is a normal web page that contains <object> with the internal viewer extension inside. The content scripts can run in the web page and modify DOM/CSS there, which is what Dark Reader does: if you inspect the page in devtools you'll see it simply inverts the colors of the object:

embed[type="application/pdf"][src="about:blank"] {
  filter: invert(100%) contrast(90%);
}

It doesn't run inside the viewer.

[Juraj M.]

Thank you @wOxxOm for the explanation :)

I see now my extension is indeed executed inside the viewer... but since all events are consumed by the embed frame and all content is encapsulated inside, it's all futile :(