Pharmacies in Paderborn: the jpoc guide
As noted on my Pharmacies in Germany page, there will always be one or two pharmacies open in Paderborn. Most of the ones in the city have somebody who speaks English but the best bet for English service is the Apotheker right on the corner at Westerntor. It's at the start of the main pedestrian zone and you cannot miss it.
Most of the Apotheker in town have a display in the window indicating which of their number is currently offering 24 hour cover. The one across the road from the main station is a good first place to look as, in the window, they have an LCD display which shows the shops on duty for the next two days. Generally, the rota means that there will be one town centre and one outlying Apotheker open all of the time.