Some times, small companies' Websites are not very accurate or up-to-date, so I called them.

From Segovia to Pedraza: at 16:45 Mon-Fri and at noon on Sat. No service on Sun.
From Pedraza to Segovia: at 7:15 Mon-Sat. No service on Sun.

Seems to be a bit uncomfortable visiting Pedraza without driving...