It makes no sense to put pdfs or docs in an images folder.
It was built like that by Joomla developers in 2005, and still used also by Us. It's just name of folder, what is difference for person who visit site, he/she download a file without deep analize of folder's name. The pdf file is important and trusted domain. For example:
/images/asset/chapter2-10.pdf
If it's very important from your point of view, you need to hire PHP developer from your city for this kind customization. I heard gossip it's possible. Check also topics on: forum.joomla.org