Wikipedia has subpages disabled in the main article namespace, which makes slashes in article names possible.

https://en.wikipedia.org/wiki/WP:TITLESLASH

I think the reason it's not appearing there is because you're looking in the "Wikipedia:" namespace, which is actually used for meta pages, not articles (e.g: https://en.wikipedia.org/wiki/Wikipedia:FAQ).

Searching under the Article namespace (which contains the actual contents of the encyclopedia), it is included: https://en.wikipedia.org/wiki/Special:PrefixIndex/PDF