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PDF Explained by John Whitington

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Chapter 6. Text and Fonts

In the previous chapter, we saw how a series of graphics operators can be used to draw content on a page, by reference to their operands and a stack-based graphics state.

In this chapter, we look at the operators and state for selecting characters from fonts and printing them on the page. Then, we see how fonts and their metrics are defined and embedded in PDF documents. Finally, we discuss the complex task of general-purpose text extraction from a document.

Text and Fonts in PDF

It would be possible to define a page description language where none of the text layout had been performed, and plain text was supplied along with boxes and columns to be filled on-the-fly, just like a desktop publishing package. Conversely, it would be possible to define a page description language without fonts or text as such at all, just relying on text being converted to outline shapes as the document is produced, having been layed out in, for example, a word-processor.

PDF adopts a middle ground—the ideas of a font and of small-scale text layout are retained, but the large-scale paragraph layout must be done in advance. This has the following advantages:

  • Complete control over layout, because large-scale layout (paragraphs, line-breaks) are the job of the program producing the PDF. The document will look as it is supposed to.

  • Predictable small-scale text layout such as strings with fixed character spacing is supported, so the position of each character need not be explicitly stated. ...

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