QQ: Why are some attachments shown WYSIWYG, but others are a link?

I've wondered this for a long time-- why some attachments are shown embedded in the Rich editor (e.g., as a picture), and others are only shown as a link.

I figured there was some technical reason for this (e.g., size), but I just noticed that the PDFs that I just scanned in show up as only links in the Rich side, but in the Markup side, are actually shown (to me, the exact opposite of expected behavior).

I did a bunch of searching, but couldn't find any other similar post. Is there an easy explanation?