用户对扫描件或者带有图片的PDF文件进行优化压缩时，可通过对图片dpi参数进行控制，从而决定最终文件的大小。示例代码如下：

public static void main(String[] args) throws PDFException {
        // Initialize library
        int error_code = Library.initialize(sn, key);
        if (error_code != e_ErrSuccess) {
            System.out.println("Library Initialize Error: " + error_code);
            return;
        }
        createResultFolder(output_path);
        String input_file =  "XXXXX.pdf";
        System.out.println("Optimized Start : Image Compression.");
        try {
         PDFDoc doc = new PDFDoc(input_file);
            error_code = doc.load(null);
            if (error_code != e_ErrSuccess) {
                System.out.println("The Doc [" +  input_file + " Error: " +  error_code);
                return;
            }
PauseUtil pause = new PauseUtil();
OptimizerSettings settings = new OptimizerSettings();
ImageSettings imageSettings = new ImageSettings();
imageSettings.setQuality(1);

//可自定义控制
imageSettings.setImageDPI(100);
imageSettings.setImageDPILimit(100);

settings.setColorImageSettings(imageSettings);
settings.setOptimizerOptions(OptimizerSettings.e_OptimizerCompressImages);
Progressive progressive = Optimizer.optimize(doc, settings, pause);
            int state = Progressive.e_ToBeContinued;
            while (state == Progressive.e_ToBeContinued) {
                state = progressive.resume();
                int rate = progressive.getRateOfProgress();
                System.out.println("Optimize progress percent: " +  rate + "%");
            }
            if(state == Progressive.e_Finished)
            {
             doc.saveAs( "Compression12.pdf", e_SaveFlagRemoveRedundantObjects);
            }
        } catch (PDFException e) {
            e.printStackTrace();
return;
        }
        System.out.println("Optimized Finish : Image Compression.");         

        Library.release();
    }