Magick.NET uses Ghostscript to read PDF files and it seems that this file cannot be read by Ghostscript. Can you add a link to your file? This might be a bug in Ghostscript.
And why are you reading the same image again? The collection already contains all the pages. You can use the MagickReadSettings class to set a different Density when you read the files in the collection.
And why are you reading the same image again? The collection already contains all the pages. You can use the MagickReadSettings class to set a different Density when you read the files in the collection.