Just a little information is needed for a better understanding as to why high lens powers are best filled in prescription eyewear.
P.D. aka pupillary distance is the measurement your optician takes to make sure that the center of the lens (which is also the location in your optical lenses where you have the clearest vision) lines up with the center of your eye. It should be taken for your distance correction and separately for your reading/near correction partly because your eyes converge when you read. This simple measurement is important and especially important as lens strength increases.
In ready-made reading glasses whose P.D. is pre-set, some people who need higher lens strengths (anything above a 3.00 lens power) may feel uncomfortable reading. This is usually the result of the increased prismatic effects of high powered lenses not lined up with your personal pupillary distance. As your lens power increases, individual measurements are best followed and custom prescription reading glasses would be a better choice. Ready-made readers may be good but not for long periods of time.