Is polar bear is stronger than a tiger?

Depends on the species of tiger. In most cases it could be said that a polar bear would be stronger than a tiger by the fact that polar bears have been known to pull narwhals and beluga whales (which often weigh over half a ton) out of the water onto the ice, whereas most tigers are only capable of taking down prey that is a little more to near equal their weight and size.

Tigers are capable of phenomenal feats of strength. There are many instances in the text. In one, a large male Bengal tiger grabbed an elephant weighing the better part of three tons by the root of the tail, and by using short jerks, pulled the elephant onto its rear. In another, a tiger had pulled a dead buffalo weighing more 1400 pounds into a swamp, and 13 men could not budge the dead animal.
A tiger in Viet Nam leaped a five foot fence carrying a young buffalo of nearly 400 pounds with it. Tigers commonly kill gaur (the huge bison of India, which may weigh 1700 pounds) and buffalo of over 1200 pounds.
Corbett's idiosyncratic "Temple" tiger carried a dead cow, weighing seven hundred pounds, up a steep hill for two miles, negotiating huge rocks and other obstructions.
To sum, overall, a 1200 polar bear would be more powerful than a 500 Bengal tiger. But pound for pound, give the edge to the cat.