Ask Mr. Smarty Plants

The first place to go to find a list of potential evergreens for your wooded Pennsylvania garden is our Native Plant Database. Use the Combination Search feature instead of Recommended Species. This will provide a bigger selection with much more choice to narrow down. The volunteers and staff at the Wildflower Center who maintain the database have partners in different regions to help with these recommended species lists based on what is easy to access in local nurseries.
Under Combination Search, select the following categories: PA, Habit – Tree, Duration – Perennial, Light Requirement – Shade and Part Shade, Soil Moisture – Moist, and Leaf Retention – Evergreen.

Some evergreen trees to consider are:

Abies balsamea (balsam fir)

Chamaecyparis thyoides (Atlantic white cedar)

Juniperus virginiana (Eastern red cedar)

Picea glauca (white spruce)

Picea mariana (black spruce)

Picea pungens (blue spruce)

Picea rubens (red spruce)

Pinus strobus (Eastern white pine)

Thuja occidentalis (arborvitae)

Since you are probably looking for an understory tree, some of the above trees may ultimately be too tall for your plans.

Hemlocks (Tsuga sp.) in the entire Northeast area are being threatened and killed by the Hemlock Wooly Adelgid (Adelges tsugae) which is a small aphidlike insect from Asia. The USDA Forest Service has an informative pest alert on their website describing the pest and treatment. 

The Pennsylvania Department of Conservation and Natural Resources also has good information about the Hemlock Wooly Adelgid on their website.

Adelges tsugae, the hemlock woolly adelgid, is a serious pest of Eastern hemlock in the northeastern states. This insect was first reported in southeastern Pennsylvania in the late 1960s and has spread to both ornamental and forest hemlocks. Adelgids are small, soft-bodied insects that are closely related to aphids. The hemlock woolly adelgid sucks sap from the young branches which results in premature needle drop and branch dieback.

These insects display several different forms during their life cycle, including winged and wingless adults. The females are oval, blackish-gray, and about 1 mm in length. Newly hatched nymphs called "crawlers" are approximately the same size, reddish-brown, and produce white, waxy, cottony or wool-like tufts that cover their bodies throughout their life. The white-woolly masses are 3 mm or more in diameter. The presence of these woolly flecks on twigs and at the base of the hemlock needles is the most obvious indicator of an infestation.

For more background information download the booklet Biology and Control of Hemlock Woolly Adelgid published by the USFS-Forest Health Technology Enterprise Team. A shorter summary is available on the Forest Health Fact Sheet on Hemlock Woolly Adelgid.

And lastly, you would like to have deer resistant evergreens.

Few plants are completely deer resistant. Several factors influence deer browsing including the density of the deer population, environmental conditions such as drought, and plant palatability. Deer tend to avoid plants with aromatic foliage, tough leathery and/or hairy or prickly leaves or plants with milky latex or sap.

The only native evergreen that fits your criteria and is somewhat deer resistant is Juniperus virginiana (Eastern red cedar).

An evergreen, aromatic tree with trunk often angled and buttressed at base and narrow, compact, columnar crown; sometimes becoming broad and irregular. Pyramidal when young, Eastern red-cedar mature form is quite variable. This evergreen usually grows 30-40 ft. but can reach 90 ft. Fragrant, scale-like foliage can be coarse or fine-cut, and varies in color from gray-green to blue-green to light- or dark-green. All colors tend to brown in winter. Pale blue fruits occur on female plants. Soft, silvery bark covers the single trunk.

Anne Bossart replied to a previous Mr. Smarty Plants Question about deer resistant trees for Pennsylvania and has some good advice ...

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 Resources decided to re-introduce them.  As you know, they co-habitat 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.

Check out Rick 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 perennials that fit the forest aesthetic.