This article shows how to reset zoom when changing orientation.

You have to change the onSizeChanged method of PDFLayoutView as following:

@Override
protected void onSizeChanged( int w, int h, int oldw, int oldh )
{
   super.onSizeChanged(w,h,oldw, oldh);
   if(m_layout != null && m_status != STA_ANNOT && w > 0 && h > 0)
   {
      m_layout.vResize(w, h);

      PDFLayout.PDFPos pdfPos = m_layout.vGetPos(0,0);
      m_layout.vZoomSet(m_layout.vGetWidth()/2, m_layout.vGetHeight()/2, pdfPos, 1);
      PDFGotoPage(m_pageno);

      if( m_goto_pos != null )
      {
         m_layout.vSetPos(0, 0, m_goto_pos);
         m_goto_pos = null;
         invalidate();
      }
   }
}

Remember that this implementation resets the view scale even if you are in zoom state.