A thought isn't always made of text. Nodes holds PDFs and images right alongside your notes, so the paper you're reading and the sketch you're referencing live in the same library as your writing.
PDFs
Bring a PDF into your library and open it in the built-in reader - no need to leave the app or hunt through Finder. Drag one onto Home or a note, or choose File ▸ Import… (⇧⌘I) to bring in several at once - see import and export. A PDF is a first-class note: you can tag it, link to it from your writing with [[…]], and keep it next to the notes that discuss it.
It's an easy way to keep source material and your own thinking in one place, connected.
Images
Images come in the same way and are tracked as their own kind of note. Drop in a diagram, a screenshot, or a photo and refer to it from anywhere in your library. You can also place an image inside a note, not just link to it - type ![[ and pick the image from the list, and it renders inline right where you're writing. A plain [[…]] points to the image; the leading ! embeds it. Pasting is quicker still: paste a screenshot or a copied image into a note and it becomes an image note of its own, embedded where the caret is.
One library, many kinds of note
Markdown, PDF, and image notes all share the same tools - the same tags, the same links, the same graph. That means you can build a single web of related material across formats, instead of scattering it across folders and other apps.
To keep it all findable, lean on tags and links. Spotlight finds them too - by name and tags, and a PDF by the text of its first few pages. To see how it fits together, open the graph.