If you're rendering a document with def_view = 3 (single page) the default behavior is to align the page on the topmost margin of the display.

In some case you would like to vertically center the page.

Look for the code below:

view.vSetLayoutPara(null, paras, m_rtol, true);

and modify with: 

view.vSetLayoutPara(null, paras,m_rtol, false);