private void OpenWendang(string strname)
        {
            ProcessStartInfo startInfo = new ProcessStartInfo();
            
            string str = System.Windows.Forms.Application.StartupPath + "\\wendang\\" + strname;
            string strs = System.IO.Path.GetFullPath(str);
            string strhzm = System.IO.Path.GetExtension(str);
            if (strhzm.ToLower() == ".doc" || strhzm.ToLower() == ".docx")
            {
                startInfo.FileName = "WINWORD.EXE";//POWERPNT
                startInfo.Arguments = strs;
            }
            else if (strhzm.ToLower() == ".pptx" || strhzm.ToLower() == ".ppt")
            {
                startInfo.FileName = "POWERPNT.EXE";//POWERPNT
                startInfo.Arguments = strs;
            }
            else if (strhzm.ToLower() == ".pdf" )
            {
                //startInfo.FileName = "JisuPdf.exe";//POWERPNT
                //startInfo.FileName = @"C:\Program Files\極速PDF閱讀器\JisuPdf.exe";
                startInfo.FileName = strs;
            }
            //startInfo.FileName = "JisuPdf.EXE";//POWERPNT
            
            try
            {
                Process.Start(startInfo);
            }
            catch
            {
                MessageBox.Show("您電腦沒有安裝office2007或者沒有安裝極速pdf軟體，無法開啟檔案請安裝再試謝謝！");
                return;

            }
        }
 

如果檔案目錄有空格會出現錯誤打不開

調整測試了下改成下面的

private void OpenWendang(string strname)
        {
            ProcessStartInfo startInfo = new ProcessStartInfo();
            
            string str = System.Windows.Forms.Application.StartupPath + "\\wendang\\" + strname;
            string strs = System.IO.Path.GetFullPath(str);
            string strhzm = System.IO.Path.GetExtension(str);
            //if (strhzm.ToLower() == ".doc" || strhzm.ToLower() == ".docx")
            //{
            //    startInfo.FileName = "WINWORD.EXE";//POWERPNT
            //    startInfo.Arguments = strs;
            //}
            //else if (strhzm.ToLower() == ".pptx" || strhzm.ToLower() == ".ppt")
            //{
            //    startInfo.FileName = "POWERPNT.EXE";//POWERPNT
            //    startInfo.Arguments = strs;
            //}
            //else if (strhzm.ToLower() == ".pdf" )
            //{
            //    //startInfo.FileName = "JisuPdf.exe";//POWERPNT
            //    //startInfo.FileName = @"C:\Program Files\極速PDF閱讀器\JisuPdf.exe";
            //    startInfo.FileName = strs;
            //}
            startInfo.FileName = strs;
            //startInfo.FileName = "JisuPdf.EXE";//POWERPNT
            
            try
            {
                Process.Start(startInfo);
            }
            catch
            {
                MessageBox.Show("您電腦沒有安裝office2007或者沒有安裝極速pdf軟體，無法開啟檔案請安裝再試謝謝！");
                return;

            }
        }