You can easily change the content type for an article, so change it from a PDF to Rich Text, or any of the other content types (e.g. video, web link, excel etc.)

This lets you change the content type without losing the history of edits to that article. Allowing you to better track the edit history for it.

Find the article you want to edit, click on the content type next to the article name and change the content type using the pop up. See the video on how to change the content type.