This may be a bit (more than a bit, actually) late, but it might still be using for someone opening this topic after a search.
Here's how I do it: one-by-one.
Code:
PdfSharp.Pdf.PdfDocument document = new PdfSharp.Pdf.PdfDocument();
foreach(string s in System.IO.Directory.GetFiles(SourceFolder, "*.tif"))
{
PdfSharp.Pdf.PdfPage page = new PdfSharp.Pdf.PdfPage();
page.Size = PdfSharp.PageSize.A4;
PdfSharp.Drawing.XGraphics xGr = PdfSharp.Drawing.XGraphics.FromPdfPage(document.Pages.Add(page));
PdfSharp.Drawing.XImage img = PdfSharp.Drawing.XImage.FromFile(s);
xGr.DrawImage(img, 0, 0);
}
document.Save(TargetName);
document.Close();
hth,
peter