Law / Will my polish national visa (D) for students allow me to enter other schengen countries? [15]

I have a polish type-D visa (multiple entries) and wanted to go to portugal for christmas. A friend told me that according to the embassy in his country this visa only allows to enter poland, but not other schengen countries, but I when I search online, it says that I can actually travel within schengen countries.

Does anyone know if I would be able to travel to portugal with my visa?