Hi Paul,
You can use PdfContentGroup.Transform
to position the imported content.
For example, try this:
var sourceSize = source.Pages[0].Size;
var destinationSize = destination.Pages[0].Size;
double yOffset = destinationSize.Height - sourceSize.Height;
group.Transform = new PdfMatrix(1, 0, 0, 1, 0, yOffset);
Also just as an FYI, you can resize that imported content, as shown in this post:
Regards,
Mario