Best practice for nested using statements?

You can remove the indention and curly brackets this way:

using (var fileStream = new FileStream("ABC.pdf", FileMode.Create))
using (var document = new Document(PageSize.A4, marginLeft, marginRight, marginTop, marginBottom))
using (var pdfWriter = PdfWriter.GetInstance(document, fileStream))
{
   // code
}

A little less verbose way to avoid the indenting:

  using (var fileStream = new FileStream("ABC.pdf", FileMode.Create))
  using (var document = new Document(PageSize.A4, marginLeft, marginRight, marginTop, marginBottom))
  using (var pdfWriter = PdfWriter.GetInstance(document, fileStream))
  {
       document.AddAuthor(metaInformation["author"]);
       document.AddCreator(metaInformation["creator"]);
       document.AddKeywords("Report Generation using I Text");
       document.AddSubject("Document subject - Describing the steps creating a PDF document");
       document.AddTitle("The document title - PDF creation using iTextSharp");
   }

As Jon Skeet pointed out, there is no need for these variables to be instance variables, as they are disposed after the using blocks anyway.

You can use local variables as shown in the code above instead.