I am closing all open things as below.
if (m_thumb != null) {
				m_thumb.thumbClose();
				m_thumb = null;
			}
			if (m_reader != null)
				m_reader.PDFClose();
			if (m_doc != null)
				m_doc.Close();
			if (mContentView != null)
				mContentView.removeAllViews();
But i don't have an Bitmap related variable.But still it gives me blank screen.When i zoom out again it show me the pdf.This happens after  when i try to zoom several pdfs.