When in doubt, benchmark! Fortunately someone else has already done this:
http://nbonvin.wordpress.com/2011/03/24/serving-small-static-files-which-server-to-use/
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.
And:
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.