It smells like progress!

All kidding aside, Steve's right about it being pulp processing related. I think it's some compounds that help bleach the pulp, and/or allow the pulp to clump. Sometimes I find a slight acridity to the smell, like that of bleach in the odour.
The smell is similar to a mix of known smells, and the smell can change slightly depending on the day.

Imagine these odours all mixed together:
wet, decomposing logs
moss
old newspapers
bleach or turpentine
rust

It's a strong smell, and sometimes can turn your appetite if you're eating or drinking something. Some people find the smell so strong that they can taste it.

almo