...
pdfViewer1.KeyDown += PdfViewer1_KeyDown;
...

private void PdfViewer1_KeyDown(object sender, KeyEventArgs e)
{
	int pageStep = pdfViewer1.AutoScrollMinSize.Height / pdfViewer1.Document.Pages.Count;
	int lineStep = pageStep / 30;
	var asp = pdfViewer1.AutoScrollPosition;

	switch (e.KeyCode)
	{
		case Keys.Down:
			pdfViewer1.AutoScrollPosition = new Point(asp.X, -asp.Y + lineStep);
			break;
		case Keys.Up:
			pdfViewer1.AutoScrollPosition = new Point(asp.X, -asp.Y - lineStep);
			break;
		case Keys.PageDown:
			pdfViewer1.AutoScrollPosition = new Point(asp.X, -asp.Y + pageStep);
			break;
		case Keys.PageUp:
			pdfViewer1.AutoScrollPosition = new Point(asp.X, -asp.Y - pageStep);
			break;
	}
}