Aspose.PDF for .NET是一種高PDF處理和解析API，用于在跨平臺應用程序中執行文檔管理和操作任務。API可以輕松用于生成、修改、轉換、渲染、保護和打印PDF文檔，而無需使用Adobe Acrobat。此外，API還提供PDF壓縮選項，表格創建和操作，圖形和圖像功能，廣泛的超鏈接功能，印章和水印任務，擴展的安全控制和自定義字體處理。

在接下來的系列教程中，將為開發者帶來Aspose.PDF for .NET的一系列使用教程，例如進行文檔間的轉換，如何標記PDF文件，如何使用表單和圖表等等。本文將介紹如何從標記的PDF中提取標記的內容。

>>Aspose.PDF for .NET更新至最新版v20.6，歡迎下載體驗。

獲得標簽的PDF內容

為了獲取帶有標簽文本的PDF文檔的內容，Aspose.PDF提供了Document Class的TaggedContent屬性 。以下代碼段顯示了如何使用“標記文本”獲取PDF文檔的內容：

// The path to the documents directory.
string dataDir = RunExamples.GetDataDir_AsposePdf_WorkingDocuments();

// Create Pdf Document
Document document = new Document();

// Get Content for work with TaggedPdf
ITaggedContent taggedContent = document.TaggedContent;

//
// Work with Tagged Pdf content
//

// Set Title and Language for Documnet
taggedContent.SetTitle("Simple Tagged Pdf Document");
taggedContent.SetLanguage("en-US");

// Save Tagged Pdf Document
document.Save(dataDir + "TaggedPDFContent.pdf");

獲取根結構

為了獲得Tagged PDF文檔的根結構，Aspose.PDF提供了ITaggedContent 接口的StructTreeRootElement和StructureElement屬性。以下代碼段顯示了如何獲取標記PDF文檔的根結構：

// The path to the documents directory.
string dataDir = RunExamples.GetDataDir_AsposePdf_WorkingDocuments();

// Create Pdf Document
Document document = new Document();

// Get Content for work with TaggedPdf
ITaggedContent taggedContent = document.TaggedContent;

// Set Title and Language for Documnet
taggedContent.SetTitle("Tagged Pdf Document");
taggedContent.SetLanguage("en-US");

// Properties StructTreeRootElement and RootElement are used for access to 
// StructTreeRoot object of pdf document and to root structure element (Document structure element).
StructTreeRootElement structTreeRootElement = taggedContent.StructTreeRootElement;
StructureElement rootElement = taggedContent.RootElement;

訪問子元素

為了訪問帶有標簽的PDF文檔的子元素，Aspose.PDF提供了 ElementList 類。以下代碼段顯示了如何訪問帶標簽的PDF文檔的子元素：

// The path to the documents directory.
string dataDir = RunExamples.GetDataDir_AsposePdf_WorkingDocuments();

// Open Pdf Document
Document document = new Document(dataDir + "StructureElementsTree.pdf");

// Get Content for work with TaggedPdf
ITaggedContent taggedContent = document.TaggedContent;

// Access to root element(s)
ElementList elementList = taggedContent.StructTreeRootElement.ChildElements;
foreach (Element element in elementList)
{
    if (element is StructureElement)
    {
        StructureElement structureElement = element as StructureElement;

        // Get properties
        string title = structureElement.Title;
        string language = structureElement.Language;
        string actualText = structureElement.ActualText;
        string expansionText = structureElement.ExpansionText;
        string alternativeText = structureElement.AlternativeText;
    }
}

// Access to children elements of first element in root element
elementList = taggedContent.RootElement.ChildElements[1].ChildElements;
foreach (Element element in elementList)
{
    if (element is StructureElement)
    {
        StructureElement structureElement = element as StructureElement;

        // Set properties
        structureElement.Title = "title";
        structureElement.Language = "fr-FR";
        structureElement.ActualText = "actual text";
        structureElement.ExpansionText = "exp";
        structureElement.AlternativeText = "alt";
    }
}

// Save Tagged Pdf Document
document.Save(dataDir + "AccessChildrenElements.pdf");

標記現有PDF中的圖像

為了標記現有PDF文檔中的圖像，Aspose.PDF提供了StructureElement Class的FindElements（）方法。您可以使用FigureElement類的AlternativeText屬性為圖形添加替代文本 。以下代碼段顯示了如何標記現有PDF文檔中的圖像：

// The path to the documents directory.
string dataDir = RunExamples.GetDataDir_AsposePdf_WorkingDocuments();
string inFile = dataDir + "TH.pdf";
string outFile = dataDir + "TH_out.pdf";
string logFile = dataDir + "TH_out.xml";

// Open document
Document document = new Document(inFile);

// Gets tagged content and root structure element
ITaggedContent taggedContent = document.TaggedContent;
StructureElement rootElement = taggedContent.RootElement;

// Set title for tagged pdf document
taggedContent.SetTitle("Document with images");

foreach (FigureElement figureElement in rootElement.FindElements(true))
{
    // Set Alternative Text  for Figure
    figureElement.AlternativeText = "Figure alternative text (technique 2)";


    // Create and Set BBox Attribute
    StructureAttribute bboxAttribute = new StructureAttribute(AttributeKey.BBox);
    bboxAttribute.SetRectangleValue(new Rectangle(0.0, 0.0, 100.0, 100.0));

    StructureAttributes figureLayoutAttributes = figureElement.Attributes.GetAttributes(AttributeOwnerStandard.Layout);
    figureLayoutAttributes.SetAttribute(bboxAttribute);
}

// Move Span Element into Paragraph (find wrong span and paragraph in first TD)
TableElement tableElement = rootElement.FindElements(true)[0];
SpanElement spanElement = tableElement.FindElements(true)[0];
TableTDElement firstTdElement = tableElement.FindElements(true)[0];
ParagraphElement paragraph = firstTdElement.FindElements(true)[0];

// Move Span Element into Paragraph
spanElement.ChangeParentElement(paragraph);


// Save document
document.Save(outFile);



// Checking PDF/UA Compliance for out document
document = new Document(outFile);

bool isPdfUaCompliance = document.Validate(logFile, PdfFormat.PDF_UA_1);
Console.WriteLine(String.Format("PDF/UA compliance: {0}", isPdfUaCompliance));