I´m using signPDF and my customers are sending pdf files with 3 pages with blank content. 

iTextSharp.text.exceptions.InvalidPdfException: PDF header signature not found.
   at iTextSharp.text.pdf.PRTokeniser.CheckPdfHeader()
   at iTextSharp.text.pdf.PdfReader.ReadPdf()
   at Cryptware.NPDFDigitSign.PDFDigitalSigner.SignPDFFile(String sourceFilename, ICertificateSelector selector, String destinationFilename)
   at CCSI_Console.NCryptokiSigner.signPDF(String Arquivo, String Destino)