I tried to read your PDF and I can create a PNG with the following code in a console application:
It looks like the problem is somewhere else in your code. Have you tried to put the code from your example in a console application?
staticvoid Main(string[] args) { MagickReadSettings settings = new MagickReadSettings(); settings.Density = new MagickGeometry(300, 300); using (MagickImageCollection images = new MagickImageCollection()) { images.Read(@"Hoja_Trabajo_2.pdf", settings); MagickImage horizontal = images.AppendHorizontally(); horizontal.Write(@"Hoja_Trabajo_2.png"); } }