I think the difference with word, excel and Tex is that because they are editable, the content must be stored in a way where the flow is expressed, and each par

I think the difference with word, excel and Tex is that because they are editable, the content must be stored in a way where the flow is expressed, and each part can be clearly broken into its parts. With PDFs, all you have is the position of lines and characters on the page. There is no flow, ordering, or semantics.