title: PDFSharp seems to be failing to open certain PDF documents
body: <p>I have a program that adds a second page to PDFs submitted to the website.  I use C# and PDFSharp.  Most documents work fine, but a few users are getting "Object reference not set to an instance of an object."</p>
  
  <pre><code>using PdfSharp;
  using PdfSharp.Drawing;
  using PdfSharp.Pdf;
  using PdfSharp.Pdf.IO;
  using PdfSharp.Drawing.Layout;
  
  PdfDocument rosterInput = PdfReader.Open(FilePath, PdfDocumentOpenMode.Import);
  
  PdfPage rpage = rosterInput.Pages[0];
  </code></pre>
  
  <p>The error occurs on the second line.  When I debug, it says PageCount = 0, which is weird because it is a 1 page document.</p>