采用PDFRender4NET

//打印机设置
PrinterSettings settings = new PrinterSettings();
settings.PrinterName = printername;
settings.PrintToFile = false;
settings.Copies = 1;
settings.PrintRange = PrintRange.AllPages;
//设置纸张大小（可以不设置取，取默认设置）
PaperSize ps = new PaperSize("Your Paper Name",595,842);
ps.RawKind = 9; //如果是自定义纸张，就要大于118

//打印设置
PDFRender.Printing.PDFPrintSettings pdfPrintSettings = new PDFRender.Printing.PDFPrintSettings(settings);
pdfPrintSettings.AutoRotate = false;            //自动旋转
pdfPrintSettings.BitmapPrintResolution = 560;   //图片打印精度                    
pdfPrintSettings.PaperSize = ps;                //纸张尺寸
pdfPrintSettings.PageScaling = PDFRender.Printing.PageScaling.FitToPrinterMarginsProportional;

pdfFile.Print(pdfPrintSettings);

代码详见： https://github.com/HiRaygo/DocumentPrint/blob/master/PdfPrint.cs

采用SumatraPDF或Acrobat Reader方案

 public static PrintEngine SumatraPDF = new PrintEngine("Sumatra PDF Reader",
            ((printerName, filePath, documentName) => 
            {
                string app = Path.Combine(Program.localPath, "SumatraPDF.exe");
                string args = string.Format("-silent -exit-on-print -print-to \"{0}\" \"{1}\"", printerName, filePath);

                Process p = new Process();
                p.StartInfo = new ProcessStartInfo()
                {
                    CreateNoWindow = true,
                    WindowStyle = ProcessWindowStyle.Hidden,
                    FileName = app,
                    Arguments = args
                };
                //if (User.isNetworkService())
                //{
                //    p.StartInfo.UserName = Program.config.serviceLogin;
                //    p.StartInfo.Password = tools.secureString(tools.Decrypt(Program.config.servicePass));
                //    p.StartInfo.Domain = Program.config.serviceDomain;
                //    p.StartInfo.LoadUserProfile = true;
                //    p.StartInfo.UseShellExecute = false;
                //}
                p.Start();

            })){};

        /// <summary>
        /// Print via external programm Acrobat Reader
        /// </summary>
        public static PrintEngine AcrobatReader = new PrintEngine("Acrobat Reader",
            ((printerName, filePath, documentName) =>
            {                
                string app = Registry.LocalMachine.OpenSubKey(
                        @"SOFTWARE\Microsoft\Windows\CurrentVersion" +
                        @"\App Paths\AcroRd32.exe"
                    ).GetValue("").ToString()
                ;
                string args = string.Format("/h /t \"{0}\" \"{1}\"", filePath, printerName);

                Process p = new Process();
                p.StartInfo = new ProcessStartInfo()
                {
                    CreateNoWindow = true,
                    WindowStyle = ProcessWindowStyle.Hidden,
                    FileName = app,
                    Arguments = args
                };
                //if (User.isNetworkService())
                //{

                //    p.StartInfo.UserName = Program.config.serviceLogin;
                //    p.StartInfo.Password = tools.secureString(tools.Decrypt(Program.config.servicePass));
                //    p.StartInfo.Domain = Program.config.serviceDomain;
                //    p.StartInfo.LoadUserProfile = true;
                //    p.StartInfo.UseShellExecute = false;
                //}
                p.Start();

})){};

代码详见： https://github.com/RepairShopr/AutoPrintr-win/blob/master/AutoPrintr/modules/PrintEngines.cs