Is pancreatic cancer usually life-threatening?

Typically. The most common type of pancreatic cancer is adenocarcinoma arising from the pancreatic duct, usually associated with a bad outcome. The reason is that, given the anatomic location, adenocarcinoma of the pancreas typically will not cause symptoms until it has developed aggressive features (such as invasion of surrounding organs or metastasis to distant organs), rendering cure unlikely.
Unfortunately, yes. The term pancreatic cancer usually refers specifically to pancreatic adenocarcinoma, the most common type. While there are other pancreatic tumors with a good prognosis, pancreatic adenocarcinoma is a very aggressive tumor. Unfortunately, most patients with this cancer are diagnosed late, at a point when achieving a cure is exceedingly rare.