The PDR consists of commercially-available package inserts from drugs prescribed in the United States and a photo index of pills. It has indices in the front of the book so you can look up drugs by generic or brand name. But don't count on finding the drug in question. Surprisingly, it omits a number of common drugs that are available as generics. Doctors use the PDR (which is shipped to our offices free each year) to identify pills (the photo guide is pretty good) and to determine dosages, interactions and side-effects.
But you can do better. As an intern I found the AHFS guide to be easier to search, more comprehensive, and better organized. You may learn more about your medicines from the AHFS but may need someone to decode the medicalese; I think that it is worth the extra effort.
Oh, and if you want to look up eye meds, you need the PDR for Ophthalmology.
I would pick up a used copy and flip through before you purchase.