I would prefer to use the "import as" mechanism to allow the user to
customize prefixes rather than hard-coding them, actually; but
currently every QML type in the QtPDF module has a hard-coded Pdf prefix.

Change-Id: I4c00e7891c58e028280599d6089a821b9e285c1a
Reviewed-by: Leena Miettinen <riitta-leena.miettinen@qt.io>