I solved by myself
for(int i = 0; i< totalPage; i++) {
int pageWidth = (int) m_doc.GetPageWidth(i);
int pageHeight = (int) m_doc.GetPageHeight(i);
Page page = m_doc.GetPage(i);
Bitmap bitmap = Bitmap.createBitmap(pageWidth, pageHeight, Bitmap.Config.ARGB_8888);
float m_ratio = 1;
com.radaee.pdf.Matrix mat = new com.radaee.pdf.Matrix(m_ratio, -m_ratio, 0, pageHeight * m_ratio);
page.RenderPrepare(bitmap);
page.RenderToBmp(bitmap, mat);
BMP bmp = new BMP();
bmp.Create(bitmap);
bmp.MulAlpha();//for transparent background.
bmp.Free(bitmap);
try {
FileOutputStream fout = new FileOutputStream("/sdcard/test"+i+".png");
bitmap.compress(Bitmap.CompressFormat.PNG, 0, fout);
fout.close();
} catch (Exception e) {
}
mat.Destroy();
page.Close();
}