Creates an empty form XObject that a PdfCanvas can draw on and draw.
public PdfFormXObject CreateFormXObject(PdfRectangle boundingBox)
The form's bounding box (BBox), in form space.
<< /Type /XObject /Subtype /Form /BBox [...] /Resources <<>> >> with no content.ISO 32000-2 §8.10.1, Table 93 (PDF 1.0). Draw on it with CreateCanvas(); paint it with DrawForm(PdfFormXObject). A mutation.