public class MyPdfViewer : PdfViewer
{
	PointF _startPt = new PointF(0, 0);
	PointF _curPt = new PointF(0, 0);
	int _shapshotPage = -1;
	bool _isSelectionStarted = false;

	protected override void OnMouseDown(MouseEventArgs e)
	{
		base.OnMouseDown(e);

		_shapshotPage = PointInPage(e.Location);
		if (_shapshotPage >= 0)
		{
			HighlightedTextInfo.Clear();
			_startPt = ClientToPage(_shapshotPage, e.Location);
			_curPt = _startPt;
			_isSelectionStarted = true;
		}

	}

	protected override void OnMouseUp(MouseEventArgs e)
	{
		base.OnMouseUp(e);
		_isSelectionStarted = false;

		if (_shapshotPage >= 0)
		{
			FS_POINTF leftTop, rightBottom;
			NormalizeSnapshotPoints(out leftTop, out rightBottom);

			var page = Document.Pages[_shapshotPage];
			var len = page.Text.CountChars;
			int lineLen = 0;
			for(int i=0; i< len; i++)
            {
				var chBox = page.Text.GetCharBox(i);
				if(IntersectsWith(chBox, new FS_RECTF(leftTop.X, leftTop.Y, rightBottom.X, rightBottom.Y)))
					lineLen++;
				else if(lineLen>0)
				{
					if (!HighlightedTextInfo.ContainsKey(_shapshotPage))
						HighlightedTextInfo.Add(_shapshotPage, new List<HighlightInfo>());
					var hInf = HighlightedTextInfo[_shapshotPage];
					hInf.Add(new HighlightInfo() { CharIndex = i - lineLen, CharsCount = lineLen, Color = Color.Red });
					lineLen = 0;
					Invalidate();
                }
            }
			_shapshotPage = -1;
		}
	}

	protected override void OnMouseMove(MouseEventArgs e)
	{
		base.OnMouseMove(e);
		if (_isSelectionStarted)
		{
			DeselectText();
			int pageIdx = PointInPage(e.Location);
			if (pageIdx != _shapshotPage)
				return;

			_curPt = ClientToPage(_shapshotPage, e.Location);
			Invalidate();
		}
	}

	protected override void OnPaint(PaintEventArgs e)
	{
		base.OnPaint(e);
		if (_shapshotPage >= 0)
		{
			var pt1 = PageToClient(_shapshotPage, _startPt);
			var pt2 = PageToClient(_shapshotPage, _curPt);
			int left = Math.Min(pt1.X, pt2.X);
			int top = Math.Min(pt1.Y, pt2.Y);
			int right = Math.Max(pt1.X, pt2.X);
			int bottom = Math.Max(pt1.Y, pt2.Y);
			e.Graphics.DrawRectangle(Pens.Red, left, top, right - left, bottom - top);
		}
	}

	private void NormalizeSnapshotPoints(out FS_POINTF leftTop, out FS_POINTF rightBotton)
	{
		leftTop = new FS_POINTF(
			Math.Min(_startPt.X, _curPt.X),
			Math.Max(_startPt.Y, _curPt.Y));
		rightBotton = new FS_POINTF(
			Math.Max(_startPt.X, _curPt.X),
			Math.Min(_startPt.Y, _curPt.Y));
	}

	public bool IntersectsWith(FS_RECTF rect1, FS_RECTF rect2)
	{
		return rect1.left < rect2.right && rect1.right > rect2.left && rect1.top > rect2.bottom && rect1.bottom < rect2.top;
	}
}