Since we treat PDF as an image format, alongside SVG, it should be possible to load icons from this format. The trick is to come up with a way to load icons by name rather than by page number. But PDF files can use arbitrary strings as page numbers anyway; so the convention could be to use the freedesktop.org icon name, e.g. document-open, as the page number; or perhaps better, use a tree of bookmarks/contents so that they can be organized hierarchically.