Created attachment 438061 [details]
<embed name="plugin" src="file:///path/to/example.pdf" type="application/pdf">
These PDFs are rendered fine, but are not accessible to VoiceOver.
https://bugs.webkit.org/show_bug.cgi?id=229556 fixed this for Mac, but not for iOS.
Note that only well-formed PDF/UA files contain the semantic information needed to generate an accessibility tree accurately. "Ordinary" PDF files (i.e. those without semantic tags) are likely to fail even in the best implementations.
However, PDF/UA in-browser should generate an accessibility tree. It does not.
This problem is not limited to iOS, and also appears on Mac Safari.
A possible workaround is stymied by another bug (223798) with aria-owns.