FREEMAIL_DOC_PDF means that your email has the following two characteristics:
- There's a doc or PDF attached.
- 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 your mail is scored with SpamAssassin's testing rules enabled. That T_ prefix marks a "testing" rule — one still being evaluated in the sandbox before it gets a real score. Same thing applies, though!)