Debug and check what is the null object in the runtime, according to exception occured line, I think
cell.Value
may have null in your DataGridView. You can add null check as below
foreach (DataGridViewCell cell in row.Cells)
{
if(cell.Value!=null)
{
pdfTable.AddCell(cell.Value.ToString());
}else
{
pdfTable.AddCell("");
}
}