System.NullReferenceException was unhandled
  HResult=-2147467261
  Message=A referência de objecto não foi definida como uma instância de um objecto.
  Source=PdfSharp
  StackTrace:
       em PdfSharp.Pdf.IO.Parser.ReadTrailer()
       em PdfSharp.Pdf.IO.PdfReader.Open(Stream stream, String password, PdfDocumentOpenMode openmode, PdfPasswordProvider passwordProvider)
       em PdfSharp.Pdf.IO.PdfReader.Open(String path, String password, PdfDocumentOpenMode openmode, PdfPasswordProvider provider)
       em PdfSharp.Pdf.IO.PdfReader.Open(String path, PdfDocumentOpenMode openmode)
       em testePDF.Form1.btProcessa_Click(Object sender, EventArgs e) em D:\Desenvolvimento\Testes\testePDF\testePDF\Form1.cs:line 73
       em System.Windows.Forms.Control.OnClick(EventArgs e)
       em System.Windows.Forms.Button.OnClick(EventArgs e)
       em System.Windows.Forms.Button.OnMouseUp(MouseEventArgs mevent)
       em System.Windows.Forms.Control.WmMouseUp(Message& m, MouseButtons button, Int32 clicks)
       em System.Windows.Forms.Control.WndProc(Message& m)
       em System.Windows.Forms.ButtonBase.WndProc(Message& m)
       em System.Windows.Forms.Button.WndProc(Message& m)
       em System.Windows.Forms.Control.ControlNativeWindow.OnMessage(Message& m)
       em System.Windows.Forms.Control.ControlNativeWindow.WndProc(Message& m)
       em System.Windows.Forms.NativeWindow.DebuggableCallback(IntPtr hWnd, Int32 msg, IntPtr wparam, IntPtr lparam)
       em System.Windows.Forms.UnsafeNativeMethods.DispatchMessageW(MSG& msg)
       em System.Windows.Forms.Application.ComponentManager.System.Windows.Forms.UnsafeNativeMethods.IMsoComponentManager.FPushMessageLoop(IntPtr dwComponentID, Int32 reason, Int32 pvLoopData)
       em System.Windows.Forms.Application.ThreadContext.RunMessageLoopInner(Int32 reason, ApplicationContext context)
       em System.Windows.Forms.Application.ThreadContext.RunMessageLoop(Int32 reason, ApplicationContext context)
       em System.Windows.Forms.Application.Run(Form mainForm)
       em testePDF.Program.Main() em D:\Desenvolvimento\Testes\testePDF\testePDF\Program.cs:line 19
       em System.AppDomain._nExecuteAssembly(RuntimeAssembly assembly, String[] args)
       em System.AppDomain.ExecuteAssembly(String assemblyFile, Evidence assemblySecurity, String[] args)
       em Microsoft.VisualStudio.HostingProcess.HostProc.RunUsersAssembly()
       em System.Threading.ThreadHelper.ThreadStart_Context(Object state)
       em System.Threading.ExecutionContext.RunInternal(ExecutionContext executionContext, ContextCallback callback, Object state, Boolean preserveSyncCtx)
       em System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state, Boolean preserveSyncCtx)
       em System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state)
       em System.Threading.ThreadHelper.ThreadStart()
  InnerException:

PdfDocument documento = PdfReader.Open(ficheiro, PdfDocumentOpenMode.ReadOnly);