70c9b9b broke a number of features of the h client that were toggled on
for PDF documents, by renaming the Annotator PDF plugin from
`Annotator.Plugin.PDF` to `Annotator.Plugin.Pdf`.

While it's certainly an ugly side-effect of a straightforward rename,
the problem is that `Guest#getDocumentInfo` (to pick one example) used
`@plugins.PDF?` to determine whether to use the PDF metadata
(provided by the PDF.js `PDFViewerApplication`).

In the absence of PDF metadata, `getDocumentInfo` falls back to using
`window.location.href`, which explains the behaviour identified
in #2506.

Fixes #2506.