It seems like PDF files have been around for a long time—in fact, if you’re young enough, they may have been around for your entire life. Nonetheless, PDFs are younger than the Mac platform you are using to edit them, and, in their short time on this planet, they have (just like you) gone through more than a few changes.
You don’t need to know most of the information in this chapter to use PDFpen productively. If you are eager to get going, feel free to jump ahead to the next chapter, Understand the Tools.
But if you like to understand why as well as how, this brief chapter gives you background on how the PDF file format evolved, what it can contain, and what you can reasonably expect to be able do with it.