Is working in an investment department belonging to a pension fund that invests employee money in stocks, governmental and private bonds, and hedge funds considered unlawful?

The ruling on your work depends on the type of stocks, bonds, and investment in hedge funds, because some stocks are prohibited, such as shares of usurious banks and usurious bonds. So, if your work in investing money is limited to permissible stocks and legitimate bonds (if they exist), then there is no harm in it; otherwise, there is.