API for drawing content on pages using the PdfDrawable.
A PdfDrawable is a drawable subclass providing a PDF-to-screen transformation
matrix to implementations via getPDFToPageTransformation(). To let the
drawable draw content on a page, create a PdfDrawableProvider returning the drawable
and register it via addDrawableProvider(PdfDrawableProvider).
| PdfDrawableManager | Interface for objects that manage a list of PdfDrawableProvider instances. |
| PdfDrawableProvider.DrawableProviderObserver | An observer interested in changes to the underlying set of drawables. |
| PdfDrawable | Drawable subclass for drawing on top of a PDF document. |
| PdfDrawableProvider | Interface for a class that can return a list of PdfDrawable instances that should be drawn on top of the document. |