I have been offered a job with a large pharmaceutical company, which, among other things, supplies materials for stem cell research. Am I able to take such a job?

Part of the answer depends on an important distinction, which many lose, in the Church’s teaching about stem cell research. The Church does not oppose all, or even most stem cell research. There are, for example, no moral issues with using stem cells harvested from adult humans, or from the umbilical cord after birth. It is only the use of stem cells acquired from human embryos, which the Church opposes, because it requires the killing of human life in order to obtain them. 
 
Hence, the company in question is not committing sin per se in supplying material for stem cell research. Only those who wrongfully use stem cells acquired from human embryos commit wrongdoing. 
 
However, let us suppose that it is clear to you that the company is certainly supplying some materials for the specific purpose of embryonic stem cell research. The morality of you accepting employment with this sort of a company would vary based on a number of factors. 
 
Let us presume, as is usually the case, that the pharmaceutical company is large and supplies a vast variety of pharmaceuticals for a wide array of medical purposes. In such a scenario taking employment with such a company would only involve you in remote material cooperation. And such associations, while not ideal, are morally permissible. 
 
However, if the position in the company that you are taking would require you to promote embryonic stem cell research to advance the sales of specific products related to embryonic stem cell research, such work would be of a more direct material cooperation. In such a case, you ought not take the job which would involve you directly advancing and cooperating in a moral evil.