As indicated by cletus, the quickest solution for you with DOMPDF is probably going to be rendering each report in a separate process. You can write a master script that calls a child script (using exec) which performs the actual rendering. As you can see in this discussion on the DOMPDF support group, it does seem to have the potential to provide a bit of a boost in performance.
It's difficult to say what's going on otherwise regarding memory usage without some kind of example that demonstrates the problem. I don't believe there is much optimization of DOMPDF and the underlaying CPDF rendering engine for multiple instances in a single script. So the font is probably being loaded into memory each time, even though it could use a static variable to cache that data.