TOPIC:

float zoom = layout.vGetZoom();
if (++zoom >= 5) {
    zoom = 1.f;
}
PDFLayout.PDFPos pdfPos = layout.vGetPos((int) x, (int) y);
layout.vZoomSet((int) x, (int) y, pdfPos, zoom);

<android.support.v4.widget.DrawerLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="match_parent"  >

    <FrameLayout
        android:layout_width="match_parent"
        android:layout_height="match_parent" >

        <ViewSwitcher
            android:layout_width="match_parent"
            android:layout_height="match_parent">
            <ProgressBar
                android:layout_width="wrap_content"
                android:layout_height="wrap_content"
                android:layout_gravity="center"/>
            <com.radaee.reader.PDFLayoutView
                android:layout_width="match_parent"
                android:layout_height="match_parent"
                android:gravity="center">
            </com.radaee.reader.PDFLayoutView>
        </ViewSwitcher>
    
    ....
  </FrameLayout>
    ....
</android.support.v4.widget.DrawerLayout>

int counter = 0;

@Override
    public boolean OnPDFDoubleTapped(PDFLayout layout, float x, float y) {
		counter++;
		if(counter % 2 == 0) {
			float currZoom = layout.vGetZoom();
			float zoomTo = ++currZoom;
			if (currZoom >= 5) {
				zoomTo = 1.0f;
			}
			final PDFLayout.PDFPos pdfPos = layout.vGetPos((int) x, (int) y);
			layout.vZoomSet((int) x, (int) y, pdfPos, zoomTo);

			m_view.invalidate();
		}
		return true;
    }