Generalizing, and documenting, the `PDFHistory`-implementation as part of the web-interfaces doesn't seem entirely necessary and in hindsight I'm not entirely sure why we need it since:
- The `PDFHistory` implementation is/was written specifically for the default viewer use-case, which is why e.g. the `simpleviewer` component example isn't using it. (While it *could* be used there, it'd need to be manually created/initialized correctly.)
- There's only *one* `PDFHistory`-implementation present (and no other viewer-component will fail without it being available), as opposed to the other web-interfaces documented in this file.
- The `PDFHistory` implementation is not even usable with e.g. the `pageviewer` component example, since it (obviously) requires a complete viewer to work. (This is in contrast to e.g. `IPDFTextLayerFactory` and `IPDFAnnotationLayerFactory`.)
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