Qt PDF is a labs module that uses pdfium to be able to render (and show) PDF's in Qt applications.
Currently it is an independent module, having their own version of pdfium. Anyhow, pdfium is nowadays hosted as part of Chromium, and is also build in Qt WebEngine.
Technically it therefore makes sense to build Qt PDF as part of Qt WebEngine.This way we only have to track one upstream, and can leverage the same infrastructure (e.g. gn/ninja).
The resulting libraries will still be independent though.