Error reading PDF

[nycjpm]

This PDF when saved to a file (and many more I can provide if needed) returns null for pages, even though it is readable in Acrobat and Firefox and Chrome.

https://www.nflgsis.com/2013/post/02/56162/Gamebook.pdf

Many other PDF's from the same site are readable. Everything newer than 2013 for example: https://www.nflgsis.com/2018/reg/02/57587/Gamebook.pdf

snippet that fails....

        using (PdfDocument pdf = PdfDocument.Open(gamebookName))
        {
            var pdfpages = pdf.GetPages();

            var pages = new List<List<string>>();
            foreach (Page page in pdfpages)   <== NULL PAGES object