"It's" a thinking reed
What's interesting is that at school, I learned (and that's what Pascal wrote) "C'est un roseau pensant", not "Il est un roseau pensant" which is found in the english translation by "he is a thinking reed" which is wrong!!
Pascal deliberately reduced Man to a "c", while in english this doesn't seem to be accepted ( an object).
The whole quote aims to first reduce man to an insignificant thing, then to emphasize the fact that "it" thinks.
It's a thinking reed.