C# - Pdfdocument - FileNotFound

Question

 public static void CreatePDFx(string path)
{
  PdfDocument outputDocumentx = new PdfDocument();
}

foreach (string pat_folder in pat_folders)
 {
         // Initially create PDFs
        Console.WriteLine("Watching this folder {0}", pat_folder);

        try
        {
               CreatePDFx(pat_folder);
        }
        catch (FileNotFoundException ex)
        {
       // Write error.
        System.IO.File.WriteAllLines(@"C:\filenotfound.txt", "Error");
         Console.WriteLine(ex);
        }
}

I have the problem that my function always thorugh an "FileNotFoundException". I have found out that it could be a problem with line 3 in the snippet. pat_folder is only a string with a path. Its not the complete code. I am using the PDFsharp class. 

Could anyone help me and tell me, why it always throw an exception? Any ideas? Thanks in advance!