`//pdf` contains the PDF plugin, its Blink-based replacement, as well as PDF utility functions that leverage PDFium. It can use low-level components that live below the content layer, as well as other foundational code like `//printing`. It should not use `//content` or anything in `//components` that lives above the content layer. Code that lives above the content layer should live in `//components/pdf`, or in the embedder. All the code here should run in sandboxed child processes. TODO(crbug.com/1220865): Remove existing `//content` dependencies.