Just to add to yamen's answer, which is perfect for images but not so much for text.
If you are trying to use this to scale text, like say a Word document (which is in this case in bytes from Word Interop), you will need to make a few modifications or you will get giant bars on the side.
May not be perfect but works for me!
using (MemoryStream ms = new MemoryStream(wordBytes))
{
float width = 3840;
float height = 2160;
var brush = new SolidBrush(Color.White);
var rawImage = Image.FromStream(ms);
float scale = Math.Min(width / rawImage.Width, height / rawImage.Height);
var scaleWidth = (int)(rawImage.Width * scale);
var scaleHeight = (int)(rawImage.Height * scale);
var scaledBitmap = new Bitmap(scaleWidth, scaleHeight);
Graphics graph = Graphics.FromImage(scaledBitmap);
graph.InterpolationMode = InterpolationMode.High;
graph.CompositingQuality = CompositingQuality.HighQuality;
graph.SmoothingMode = SmoothingMode.AntiAlias;
graph.FillRectangle(brush, new RectangleF(0, 0, width, height));
graph.DrawImage(rawImage, new Rectangle(0, 0 , scaleWidth, scaleHeight));
scaledBitmap.Save(fileName, ImageFormat.Png);
return scaledBitmap;
}