Because PDX is so far away from JFK, we recommend simply booking a flight. Other options to PDX do exist, but going by plane will save you valuable time. Book as soon as you can, as there may be limited availability.
On average, the best time to book a flight for the winter is 5 - 6 days in advance. Including seasonlity, while spring flights should be booked about three months prior to, summer about 5 weeks in advance, and fall and winter flights about 6 weeks in advance. Shuttle service may be available to PDX. If you're traveling with a large group, you should probably take a shuttle as it will reduce the price per person. Alternatively, there are shared shuttle services available. With this option, you'll want to book as soon as possible to allow the shuttle service enough time to coordinate your pickup and dropoff.
While buses have recently gotten bad coverage because of cleanliness of bus stations or winter time accidents, they are can be very convenient for travel. The best part about traveling by bus to PDX is that it can be very cheap. That's because they usually have basic accomodations - WiFi and a restroom. Some of the JFK/PDX buses' coverage has been overstated. The real thing to consider when taking a bus from JFK is to consider the time of travel and the density of the cities that you're traveling to. If you are traveling by bus between JFK and PDX during high traffic times, you can expect to have your trip altered significantly due to traffic. Further, if you are traveling during winter, the bus schedule might be thrown off due to inclimate winter weather. Be sure to plan accordingly!
The plus side of taking the train from JFK to PDX? The accomodations, scenery, and lack of PDX traffic can usually be on point. Moreover, you can usually expect your average concessions - such as food and drinks. Most trains have WiFi these days as well. The worst part about trains are the price. While the train to PDX is often cheaper than flying, they are generally more expensive than buses. Train prices are usually set and do not vary as much as bus prices or flights or so the likelihood of finding discounts is pretty rare.