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iText是一个支持创建和操作PDF文档的PDF库，通过动态生成和操作PDF文档，iText能帮助开发者进一步提升网站和应用程序的性能，它也同时支持Java和C#。

创建PDF

有了Document和PdfWriter类，你可以从数据库，XML文件或者其他数据源开始创建PDF文档，可以通过以下三种途径完成：

• 使用高层次的对象，如Chunk、Phrase、Paragraph、List等等。这些对象通常被称为iText的基础构造模块。
• 使用低层次的功能。这是通过PdfContentByte类完成的，它包含了一系列方法，可以映射到Adobe成像模型中的每一个操作员和运算对象。这个类还包含大量便捷的方法，用以绘制绝对定位的圆弧，圆和矩形。
• 使用PdfGraphics2D类，它是iText对抽象类Graphics2D的实现（不适用于iTextSharp）。
  

iText附带了大量类，可支持ecnryption、不同的图像类型、色彩空间和字体，增强PDF文件的可访问性，支持Flash应用程序集成到PDF中等。

iText可以将XML或者XHTML/CSS文件转换为PDF文件（使用XML Worker）。iText不支持将文件从一种格式转换为另一种格式，不支持将Word文档转换为PDF文档。

更新PDF

当你需要一个PDF阅读器来访问现有的PDF文档，你可以使用这个阅读器，结合PdfStamper标记当前PDF文档中额外的内容，如页码、水印和注释等等。PdfStamper类还支持填写交互表单。

你也可以通过PdfCopy，PdfSmartCopy，PdfCopyFields，甚至使用结合PdfWriter或PdfStamper的PdfImportedPage类来拆分和合并PDF文档。iText不支持将PDF文档转换为另外格式的文档，但是如果PDF文档被标记或者包含结构树，你可以将它转换为XML文件。

iText还可以对现有的PDF文档进行签名和加密。

阅读PDF

iText不是PDF查看器，不能将PDF转换为图像，也不能打印PDF文档，但是PdfReader能让你访问一些特定类，这些类创建PDF文档和页面中的内容流。内容流能被解析，并且如果它们不是以栅格化的文本添加到页面中的，你可以将页面转换为纯文本。同时，iText不支持OCR。

何时使用iText

一般情况下，iText适用于有以下需求的项目中：

• 内容无法提前利用：取决于用户的输入或实时的数据库信息。
• 由于内容，页面过多，PDF文档不能手动生成。
• 文档需在无人参与，批处理模式下自动创建。
• 内容被定制或个性化；例如，终端客户的名字需要标记在大量的页面上。

Creating PDFS

With the Document and the PdfWriter class, you can create PDF documents from scratch from a database, an XML file, or any other data source. You can do this in three different ways:

• using high-level objects such as Chunk, Phrase, Paragraph, List, and so on. These objects are often referred to as iText's basic building blocks.
• using low-level functionality. This is done with PdfContentByte, a class that consists of a series of methods that map to every operator and operand available in Adobe's imaging model. This class also has numerous convenience methods to draw arcs, circles, rectangles and text at absolute positions.
• Using PdfGraphics2D which is iText's implementation of the abstract Graphics2D class in Java (not available in iTextSharp).

iText ships with a plethora of classes that support ecnryption, different image types, color spaces, fonts. There's functionality to enhance the accessibility of the PDF file, support for the integration of Flash apps into the PDF, and so on.

iText can convert an XML or an XHTML/CSS file to PDF (using XML Worker). Converting documents from one format to another is outside the scope of iText. And no: iText does not convert Word documents to PDF!

Updating PDFS

You always need a PdfReader instance to access an existing document. You can use this reader in combination with PdfStamper to stamp extra content on the existing PDF document: page numbers, a watermark, annotations, and so on. PdfStamper is also the class you'll use to fill out interactive forms. iText has almost complete support for AcroForms, but as soon as you have a form involving the XML Forms Architecture, the possibilities are limited.

You can split and merge PDF documents with PdfCopy, PdfSmartCopy, PdfCopyFields, and even using PdfImportedPage objects in combination with PdfWriter or PdfStamper.
Converting a PDF document to another format is outside the scope of iText, but you can convert a PDF to XML if the PDF was tagged and contains a structure tree. Depending on how the PDF was created, you can also extract plain text from a page.

iText can also be used to sign existing PDF documents, as well as to encrypt them.

Reading PDFS

iText isn't a PDF viewer, iText can't convert PDF to an image, nor can iText be used to print a PDF, but the PdfReader class can give you access to the objects that form a PDF document and to the the content stream of each page. This content stream can be parsed and if the content wasn't added as rasterized text, you can convert a page to plain text. Note that iText doesn't do OCR.

When to use iText?

Typically, iText is used in projects that have one of the following requirements:

• The content isn't available in advance: it's calculated based on user input or real-time database information.
• The PDF files can't be produced manually due to the massive volume of content: a large number of pages or documents.
• Documents need to be created in unattended mode, in a batch process.
• The content needs to be customized or personalized; for instance, the name of the end user has to be stamped on a number of pages.