Ask Mr. Smarty Plants

Unfortunately, there is no such thing as a deer proof evergreen ... especially in Pennsylvania.  One of life's ironies is that deer were hunted to extinction in Pennsylvania by the early 1900's.  At that point someone in the Department of Natural Resouces decided to re-introduce them.  As you know, they co-habitate happily with humans in a suburban environment but their natural predators (wolves and big cats) do not. 

That being said, deer do have preferences and they seem to be regional.  On one street they'll devour a plant and leave it alone on the next.  So my best advice is to inquire locally (neighbors, nurseries and Master  Gardener hotlines) as to what gardeners have had the most success with. In general, though, they prefer arborvitae and fir to hemlock and pine.  Hemlock is not a good choice in your area, however, as it is quite susceptible to woolly adelgid and short-lived.  You may find some helpful information in this article published by Horticulture Magazine and this link to a publication by Rutgers University.

Some gardeners enclose their newly-planted trees within a heavy, wire enclosure -- sort of a tree-sized "tomato hoop."  Not only can a hoop deter browsing, but it will prevent bucks from rubbing the bark off your young trees when they're shedding velvet from their antlers in the fall.  If erecting a hoop or building a fence at least 8 feet tall to exclude them is not a possibility, I recommend trying to ignore the problem.  You are living in their habitat and fighting a losing battle.

You might consider foregoing a solid evergreen screen and instead erect a fence and mixed deciduous hedgerow.  Your privacy will not be as complete in the wintertime but will be effective in the seasons where you are outside.  It will be more resistant to deer browse, less obvious when it is attacked and will be more attractive than a solid green wall between you and the woods.

Check out Ricke Darke's book, "The American Woodland Garden" for inspiration.  While written from a compelling and fresh perspective, this book never strays from the realistic concerns of the everyday gardener.  An alphabetical listing of woodland plants offers useful advice for every garden, emphasizing native trees, shrubs, vines, ferns, grasses, sedges and flowering perennilas that fit the forest aesthetic. 

It is a wonderful book that will help you see your "yard backs up to woods" in a new light.