I got it, mostly from looking at the following question. I needed to use the Doc.PageNumber to access each page of the pdf. Here's the print page event where I changed the code.
void pd_PrintPage(object sender, PrintPageEventArgs e)
{
_currentItemIndex++;//added index to keep track of page. default to 1
appendedDocs.PageNumber = _currentItemIndex;//set to current page for printing
XRect cropBox = appendedDocs.CropBox;
double srcWidth = (cropBox.Width / 72) * 100;
double srcHeight = (cropBox.Height / 72) * 100;
double pageWidth = e.PageBounds.Width;
double pageHeight = e.PageBounds.Height;
double marginX = e.PageSettings.HardMarginX;
double marginY = e.PageSettings.HardMarginY;
//center it
double x = (pageWidth - srcWidth) / 2;
double y = (pageHeight - srcHeight) / 2;
x -= marginX;
y -= marginY;
RectangleF rect = new RectangleF((float)x, (float)y, (float)srcWidth, (float)srcHeight);
appendedDocs.Rect.SetRect(cropBox);
int rez = e.PageSettings.PrinterResolution.X;
appendedDocs.Rendering.DotsPerInch = rez;
Graphics g = e.Graphics;
using (Bitmap bitmap = appendedDocs.Rendering.GetBitmap())
{
g.DrawImage(bitmap, rect);
}
e.HasMorePages = _currentItemIndex < appendedDocs.PageCount;//check for more pages.
}
Feel silly having asked this question and then answering myself. But it feels good knowing that this question is now out there for anyone else that gets stuck on the issue.