title: PdfBox flatten pdf does not remove acroform elements
body: <p>I have a pdf with a lot of acroforms, I do some manipulation on it which results in a new pdf. 
  So I have PDF-1 (which is the original one )and PDF-2 (just a duplication of PDF-1), now I want to merge them. Both PDFs have some acroforms for example: field_a, field_2...</p>
  
  <p>Before I merge them I flatten PDF-1, because I only want to have acrofields from PDF-2. When I check then my new merged PDF I can see that there are no visible fields on on the pages from PDF-1 and there are fields on pages of fields of PDF-2. At the first look it seems ok, but when I inspect the fields I can see  that the merger has renamed all the fields for PDF-2 e.g. field_a_dummy123, field_b_dummy232 ...</p>
  
  <p>It seems to me, that flattening does not remove the fields and thats why the PDFMerger from PDFBox will rename the fields for PDF-2 because acrofields need to be unique. Is there a way to completely remove the acroforms of PDF-1? </p>
  
  <pre><code>@Test
  public void flattenAndMerge() throws IOException {
      File testForm = new File(classLoader.getResource("./TestForm.pdf").getFile());
  
      byte[] testFormAsByte = Files.readAllBytes(testForm.toPath());
      byte[] testFormAsByte2 = Files.readAllBytes(testForm.toPath());
  
      PDDocument pdf1 = PDDocument.load(testFormAsByte);
      PDAcroForm acroform = pdf1.getDocumentCatalog().getAcroForm();
      acroform.flatten();
      Path flattendedPdf = Files.createTempFile("flatten", ".pdf");
      pdf1.save(flattendedPdf.toFile());
  
  
      PDFMergerUtility merger = new PDFMergerUtility();
      merger.addSource(new ByteArrayInputStream(Files.readAllBytes(flattendedPdf)));
      merger.addSource(new ByteArrayInputStream(testFormAsByte2));
      merger.setDestinationFileName("./build/flattenAndMerge.pdf");
      merger.mergeDocuments(MemoryUsageSetting.setupMainMemoryOnly());
  
  }
  </code></pre>
  
  <p>I am using PDFBox 2.0.8.</p>
  
  <p>This is the input file: <a href="https://ufile.io/6etxp" rel="nofollow noreferrer">https://ufile.io/6etxp</a>
  Here is the result of the test: <a href="https://ufile.io/bh94n" rel="nofollow noreferrer">https://ufile.io/bh94n</a></p>
  
  <p>As I could see the problem only occures with checkboxes, normal text fields will be removed correctly</p>