PDF Attachment to Markdown (.md)

When I attach an image (drag-and-drop) to a markdown file, the image appears. If I click on the image, it appears above the document.

When I attach a PDF in the same way, the PDF thumbnail appears. However, if I click on the PDF, it shows an outline of where it could be filled in, but there’s an icon that shows it’s not available.

Is this because it’s trying to treat the PDF like an image?

Are PDF’s allowed to be attached?