This article shows how to create a thumbnail of a required page.

The code runs with RadaeePDF SDK on Android and Server and it's written in Java.

public Bitmap getThumbnailFromPage(String m_path, String m_pwd, int pageno, int thumbWidth, int thumbHeight)
{
   Bitmap bmp = null;

   Document doc = new Document();
   if( doc.Open(m_path, m_pwd) == 0 )
   {
      Page page = doc.GetPage(pageno);
      try
      {
         bmp = Bitmap.createBitmap( thumbWidth, thumbHeight, Bitmap.Config.ARGB_8888 );
         bmp.eraseColor(0);
         if( !page.RenderThumb(bmp) )
         {
            float w = doc.GetPageWidth(0);
            float h = doc.GetPageHeight(0);
            float ratiox = thumbWidth/w;
            float ratioy = thumbHeight/h;
            if( ratiox > ratioy ) ratiox = ratioy;
            Canvas canvas = new Canvas(bmp);
            Paint paint = new Paint();
            paint.setARGB(255, 255, 255, 255);
            canvas.drawRect((thumbWidth - w * ratiox)/2, (thumbHeight - h * ratiox)/2,
                  (thumbWidth + w * ratiox)/2, (thumbHeight + h * ratiox)/2, paint);
            Matrix mat = new Matrix( ratiox, -ratiox, (thumbWidth - w * ratiox)/2, (thumbHeight + h * ratiox)/2 );
            page.RenderPrepare((Bitmap)null);
            page.RenderToBmp(bmp, mat);
            mat.Destroy();
            bmp.recycle();
            bmp = null;
         }
      }
      catch(Exception e)
      { e.getMessage(); }
      page.Close();
      doc.Close();
   }
   return bmp;
}