I'd like to prevent hotlinking of pdfs in a certain website directory without imposing a site-wide ban, and redirect any direct traffic to a given page. I suspect htaccess is my friend for this but don't know how I should define these rules.
Try:
RewriteEngine On
RewriteCond %{HTTP_REFERER} !^https?://example\.com/ [NC]
RewriteRule ^folder/ /given-page.html [L,R]
Here, any request for anything in the folder /folder/
that is not referred to by your site (example.com) will be redirected to /given-page.html
.