You need to set the density when you read the PDF that you have just created, otherwise it will use 72 dpi as the default.
I would however advise you to not use a PDF file as an intermediate format. This format use the GhostScript program and creates PNG files on disk. You are probably better of to use TIFF or PNG as the intermediate format.
MagickReadSettings settings = new MagickReadSettings(); settings.Density = new MagickGeometry(300, 300); MagickImage magickImageDestination = new MagickImage(destinationStream, settings);