There are many types of swelling. The main abdominal swelling in cancer patients is a swelling know as ascities.
This is when there is too much of the fluid used as a lubricant between the peritomeum and the internal organs, allowing them to move freely across each other. A build up of this fluid is know as ascities to medical professionals. I nursed a patient recently who drained 10 liters. This causes for ascities are mainly to do with blockages of drainage systems such as lymph glands as well as other surrounding tissues.
The swelling of the abdomen may also be related to lymphodema but this is more common in the arms, legs and feet.
Cancer spreads very quickly and when patients are in the end stages of their cancer, secondary tumors are often not investigated. Pancreatic cancer often spreads to the linings of the abdomen, this will cause build up of fluid, similarly blockage of drainage systems.
When bile ducts become blocked with cancer cells this will also cause swelling because of the trauma it causes to the local areas.
Pancreatic cancer is often exceptionally painful and patients need to take large amounts of painkillers to control their pain. These medications often cause constipation and bowel obstruction which will also cause swelling of the lower abdominal area.
I hope this has been of some use to you, it is difficult to explain causes of swellings because each patient has a differnt cause and the swelling could be caused by any number of things, but hopefully this has helped answer your question.