虚拟打印机的实现，功能：运行该程序之后，系统打印机列表中出现一个新的打印机（虚拟的）。用户可以选用该打印机进行打印，打印的结果是文件（ ps格式，PDF格式，txt格式。）-Implementation of the virtual printer, function: run the program, the system appears in the printer list, a new printer (virtual). Users can select the printer to print, print the results file (ps format, PDF format, txt format.)