Others will probably suggest adapting an existing plugin with work-arounds for a ebook (PDF) collection. There is a plugin for a Media collection but don't know if it will allow ebooks (in your case PDF).
Am gonna throw out an idea ... which may/may not be desired, but might be exactly what you need ... another server built for ebooks ... Calibre
clip from above link:
"Share and backup your library easily
calibre comes with a handy web server to host
your library. In just a few moments you can safely
share all (or just some) of your e-books with anyone
you choose or access them yourself from anywhere, using any
device you like. With the server, you can read the books
in your calibre library on any phone/tablet using just
a browser. It even works offline.
calibre has also some useful backup and import/export
features that will guarantee the safety and portability of
your book collection with minimal fuss."
Truth in sharing ... have run Calibre on a Mac Laptop ... primarily for converting ... never used the 'handy web server' so can't speak to just how 'handy' that is ... also never explored interfacing with Moodle.
OR, one might be able to take advantage of what Google Drive (or similar) offers.
From your Moodle, a simple link to your shared collection.
'spirit of sharing', Ken