The practical approach: find others doing similar things (on the Internet there are always others doing similar things) and see if you can find out more about how they work.
Here are some other public domain ebook "libraries" that I know about
:
Project Gutenberg you will already know about.
But did you know about sites like:
Project Gutenberg of Australia (PGA)? That site also links to this
Roy Glashan's Library.
Adelaide University has been doing this sort of stuff for a long time.
Do some Internet searches of these and similar sites and see if you can find out what sorts of places link to them - perhaps they are the sorts of places that would link to yours too. The fact that PGA linked to Roy Glashan's suggests that some of these libraries are happy to collaborate - so perhaps your emphasis on PDF might complement these sites that concentrate on HTML presentation. You might try contact some of the smaller ones that show an inclination to link to alternative resources (such as PGA).
(Given that these have been active for many years, you might also take some lessons from them about what works and what doesn't work on a website for public domain ebooks. eg: I didn't come across any library cards at these sites.)