In the exported PDF document, the font size of the header and footer is smaller than the font of the data cells, even though the specified font is the same size.
To reproduce set the same font to the header and data cells:

pdfExport.HeaderFont.Font = new Font("Segoe UI", 10f);

private void PdfExport_CellFormatting(object sender, PdfExportCellFormattingEventArgs e)
{
    e.CellElement.Font = new Font("Segoe UI", 10f); 
}