Expands the configured boundary margin so that a document smaller than the view is aligned per underflowAnchor.
underflowAnchor
The viewport size.
The zoom factor.
The laid-out document size.
The boundaryMargin value (EdgeInsets).
Optional
The underflowAnchor value (PdfPageAnchor).
The resulting EdgeInsets.
Expands the configured boundary margin so that a document smaller than the view is aligned per
underflowAnchor.