FREEMAIL_DOC_PDF

FREEMAIL_DOC_PDF means that your email has the following two characteristics:

1. There's a doc or PDF attached.
2. It's coming from a "free" email provider (hotmail, Gmail, and so on.)

This, as you might imagine, is a rule that hits a lot of emails — even completely genuine ones! Which is why most services that implement SpamAssassin don't weight it very highly.

(P.S. you might also run into T_FREEMAIL_DOC_PDF if you're using or testing against an older version of SpamAssassin. That T_ prefix stands for "trunk" — the "freemail" series of heuristics is a relatively recent one. Same thing applies, though!)