如何有效地使用 PDFBox 3.0 标记 PDF 文档是否符合可访问性，并确保其符合 WCAG 和 PDF/UA 标准？

问：

我目前正在做一个项目，我需要让残障人士可以访问 PDF 文档。我正在使用 PDFBox 3.0 创建这些 PDF，但我在正确标记 PDF 以满足可访问性合规性方面面临挑战，我已经做了其他所有事情来使它们符合要求（XMP 元数据、标记字典等）。

我按照已经发布在堆栈溢出上的这两个指南进行操作，但没有结果。我预计这些标签将反映在文档中，但是在Acrobat中测试时，仅反映逻辑结构下的顶级标签

这是目前的代码，请注意，为了保持简洁，我已经剥离了一些布局代码。

object GeneratorImpl {

  protected class PDFContext {
    val document: PDDocument = new PDDocument()
    var page: PDPage = new PDPage(PDRectangle.A4)

    page.getCOSObject.setItem(COSName.STRUCT_PARENTS, COSInteger.get(0))
    var contentStream: PDPageContentStream = new PDPageContentStream(document, page)
    var currentY: Float = 0

    val MARGIN = 50f
    var unsupportedCharsFound = false

    page.setMediaBox(PDRectangle.A4)
    document.addPage(page)

    //Set Document Language
    document.getDocumentCatalog.setLanguage("en")
    document.getDocument.setVersion(1.7F)

    //Set Document Viewer Preferences and add metadata
    document.getDocumentCatalog.setViewerPreferences(new PDViewerPreferences(new COSDictionary()));
    document.getDocumentCatalog.getViewerPreferences.setDisplayDocTitle(true);

    //Mark the PDF as Tagged
    private val markInfo = new PDMarkInfo()
    markInfo.setMarked(true)
    document.getDocumentCatalog.setMarkInfo(markInfo)

    addXMPMetadata(document)

    val structureTreeRoot = new PDStructureTreeRoot()
    document.getDocumentCatalog.setStructureTreeRoot(structureTreeRoot)

    val root = new PDStructureElement(StandardStructureTypes.DOCUMENT, structureTreeRoot)
    val dictionary = new COSDictionary()
    dictionary.setItem(COSName.STRUCT_PARENTS, COSInteger.get(0))

    var pageStructureElement = new PDStructureElement(dictionary)
    root.appendKid(pageStructureElement)

    var currentMCID = 0
    var currentMarkedDictionary = new COSDictionary()
    currentMarkedDictionary.setInt(COSName.MCID, currentMCID)

    var nums = new COSArray()
    var numDictionaries = new COSArray()
  }

  def generateCertificatePDF(fields: Seq[Field]): Unit = {
      var pdfContext = createContext()

      if (fields.isEmpty) throw NonRetryableException("No fields supplied to PDF generator")

      for (field <- fields) {
        field match {
          case header: Header => {
            pdfContext = addTitle(header.contents, pdfContext.ROBOTO_BOLD, 20)(pdfContext)
          }
          // Stripped out the other cases for simplicity
          case _ => throw NonRetryableException("Unknown field type in list. Cannot generate PDF without matching generation logic.")
        }
      }

      pdfContext.contentStream.close()
      addParentTree(pdfContext)

      pdfContext.document.save(new File("./test.pdf"))
      val outputStream = new ByteArrayOutputStream()
      pdfContext.document.save(outputStream)

      outputStream.close()
      pdfContext.document.close()
  }

  private def addTitle(text: String, font: PDType0Font, fontSize: Float)(implicit context: PDFContext): PDFContext = {
    if (text.nonEmpty) {
      setDocumentTitle(text)
      addContentToParent(COSName.P, StandardStructureTypes.H, context.page, context.pageStructureElement)
      addCenteredTextBlock(text.toUpperCase, fontSize, font)
    }

   context
  }

  private def addCenteredTextBlock(text: String, fontSize: Float, font: PDType0Font)(implicit context: PDFContext): PDFContext = {
    var localContext = context
    localContext.contentStream.setFont(font, fontSize)

    // 2 * context.MARGIN for left and right margin * 2 for extra padding
    val maxCentreWidth = localContext.page.getMediaBox.getWidth - (2 * localContext.MARGIN * 2)
    localContext.currentY = localContext.page.getMediaBox.getHeight - (2 * localContext.MARGIN)

    val startX = (localContext.page.getMediaBox.getWidth - maxCentreWidth) / 2f
    val startY = localContext.currentY
    var currentY = startY

    val lines = wrapText(text, font, fontSize, maxCentreWidth)
    for (line <- lines) {
      if (localContext.currentY - fontSize <= localContext.MARGIN) {
        localContext = addPage(fontSize, font)
      }

      val textWidth = fontSize * font.getStringWidth(line) / 1000F
      val currentX = startX + (maxCentreWidth - textWidth) / 2f

      localContext.contentStream.beginText()
      localContext.contentStream.newLineAtOffset(currentX, currentY)

      localContext.contentStream.beginMarkedContent(COSName.P, PDPropertyList.create(context.currentMarkedDictionary))
      localContext.contentStream.showText(line)
      localContext.contentStream.endMarkedContent()
      setNextMarkedDictionary

      if (line.contains("\n") || line.contains("\\n") || line.equals("")) {
        currentY -= fontSize
        localContext.contentStream.beginMarkedContent(COSName.ARTIFACT, PDPropertyList.create(context.currentMarkedDictionary))
        localContext.contentStream.newLineAtOffset(currentX, currentY)
        localContext.contentStream.endMarkedContent()
        setNextMarkedDictionary
      }

      localContext.contentStream.endText()

      currentY -= fontSize
      localContext.currentY = currentY

    }

    localContext
  }

  private def setNextMarkedDictionary(implicit context: PDFContext): Unit = {
    context.currentMCID += 1
    context.currentMarkedDictionary = new COSDictionary()
    context.currentMarkedDictionary.setInt(COSName.MCID, context.currentMCID)
  }

  //Add a structure element to a parent structure element with optional marked content given a non-null name param.//Add a structure element to a parent structure element with optional marked content given a non-null name param.
  private def addContentToParent(name: COSName, structureType: String, currentPage: PDPage, parent: PDStructureElement)(implicit context: PDFContext) = {
    //Create a structure element and add it to the current section.
    var structureElement: PDStructureElement = null
    if (structureType != null) {
      structureElement = new PDStructureElement(structureType, parent)
      structureElement.setPage(currentPage)
    }
    //If COSName is not null then there is marked content.
    if (name != null) {
      //numDict for parent tree
      val numDict = new COSDictionary()
      numDict.setInt(COSName.K, context.currentMCID)
      numDict.setItem(COSName.PG, currentPage.getCOSObject)
      if (structureElement != null) {
        if (!COSName.ARTIFACT.equals(name)) structureElement.appendKid(new PDMarkedContent(name, context.currentMarkedDictionary))
        else structureElement.appendKid(new PDArtifactMarkedContent(context.currentMarkedDictionary))
        numDict.setItem(COSName.P, structureElement.getCOSObject)
      }
      else {
        if (!COSName.ARTIFACT.equals(name)) parent.appendKid(new PDMarkedContent(name, context.currentMarkedDictionary))
        else parent.appendKid(new PDArtifactMarkedContent(context.currentMarkedDictionary))
        numDict.setItem(COSName.P, parent.getCOSObject)
      }
      numDict.setName(COSName.S, name.getName)
      context.numDictionaries.add(numDict)
    }
    if (structureElement != null) parent.appendKid(structureElement)
    structureElement
  }

  //Adds the parent tree to root struct element to identify tagged content//Adds the parent tree to root struct element to identify tagged content

  private def addParentTree(implicit context: PDFContext): Unit = {
    val dict = new COSDictionary()
    context.nums.add(context.numDictionaries)
    var i = 1
    while (i < 1) {
      context.nums.add(COSInteger.get(i))

      i += 1
    }
    dict.setItem(COSName.NUMS, context.nums)
    val numberTreeNode = new PDNumberTreeNode(dict, dict.getClass)
    context.document.getDocumentCatalog.getStructureTreeRoot.setParentTree(numberTreeNode)
    context.document.getDocumentCatalog.getStructureTreeRoot.appendKid(context.root)
  }

}