TOPIC:

    public class PDFSimpleOnScaleGestureListener extends
    SimpleOnScaleGestureListener {

       @Override
       public boolean onScale(ScaleGestureDetector detector) {
        return true;
       }
    
       @Override
       public boolean onScaleBegin(ScaleGestureDetector detector) {
           mActivityViewScaleCallback.onPDFViewScaleCallback();
        return false;
       }
    
       @Override
       public void onScaleEnd(ScaleGestureDetector detector) {
       }

    }

//With an Interface for callback:

    /**
     * Callback for PDFViewerActivity
     */
    public interface OnPDFViewScaleCallback {
        /**
         * Callback when user starts to zoom pdf view
         */
        public void onPDFViewScaleCallback();
    }

//i use pdfview in a fragment so my activty implements the interface and to set the callback

    @Override
    public void onAttach(Activity activity) {
        super.onAttach(activity);
        mActivityViewScaleCallback = (OnPDFViewScaleCallback) activity;
        scaleGestureDetector = new ScaleGestureDetector(activity, new PDFSimpleOnScaleGestureListener());
    }

//and then i set the scaleGestureDetector to readercontroller (when view is drawn)

            mPdfView.setOnTouchListener(new OnTouchListener() {
                
                @Override
                public boolean onTouch(View v, MotionEvent event) {
                    scaleGestureDetector.onTouchEvent(event);
                    return false;
                }
            });
        }


//and in my activity I implement the interface and every time the user starts to scale the callback method is called
    @Override
    public void onPDFViewScaleCallback() {
       //TODO -> Your stuff
    }