Refer:

http://atashbahar.com/post/Converting-Multipage-TIFF-image-to-PDF.aspx

http://social.msdn.microsoft.com/forums/en-US/csharpgeneral/thread/fb9c29bd-bb4c-4bb9-b6b9-11fc5a71cfa5

http://stackoverflow.com/questions/1982028/bad-performance-convert-tif-to-pdf-using-itextsharp

http://www.codeproject.com/KB/files/tifftopdf.aspx

http://blog.codebeach.com/2008/02/convert-multipage-tiff-to-pdf-in-net.html