When in doubt, benchmark! Fortunately someone else has already done this:
Conclusion? Allow me to quote:
G-WAN seems again to perform a lot better than the other servers. Nginx always performs slightly better than Lighttpd, while Apache
Traffic server is very similar to Lighttpd in term of performance.
Regarding the resources used by each server, Nginx is the winner in
term of memory usage, as the amount of memory does not increases with
the number of concurrent clients. G-WAN requires 2 times less CPU than
the other servers.
The proof is in the numbers. Nginx is second best at requests/second, average for cpu usage, and best for memory usage. So yes, nginx is definitely a good server for statically serving standard size PDFS.