Wilson, yes, poster board can work, but it depends on the surface. 😊

If the poster board has a smooth, slightly toothy surface, you should be able to draw on it without any major issues. The main thing to avoid is glossy or coated poster boards, as graphite tends to smudge and doesn’t layer very well on them.
To be honest, I’ve drawn on everything from chart paper to cardstock and even cardboard when testing ideas. The paper is important, but not as important as many people think. If the surface accepts graphite and erasing reasonably well, you can create great artwork on it.
If possible, buy a single sheet first and do a small shading test before committing to a larger drawing.