Unsure, I use a plugin called Redirection which allows redirects and passthroughs (where it shows the url you choose but the page at the target url). It works really nicely but it doesn't allow passthroughs for pdf files.
Ok so "passthrough" is their name for rewrite.
It's not just "showing" the target url with a redirect, it's actually going to that page instead. The rewrite engine is basically sending back a message "go find this content at <TARGET_URL>" and the browser then requests that other page.
With a rewrite, the rewrite engine basically tells the server "instead of <SLUG_URL> give me the page <TARGET_URL>" and then passes it back to the browser as <SLUG_URL>