You got the following MagickReadSettings in your example project:
When you set the Format to MagickFormat.Png you are telling Magick.NET/ImageMagick that the input buffer should be read as a .PNG file. But since your input is PDF it fails to read the file.
MagickReadSettings settings = new MagickReadSettings { Density = new PointD(150, 150), Format = MagickFormat.Png, ColorSpace = ColorSpace.GRAY };