I recently received a 12-page PDF (yay, a relic document format from 1996!) and had to redact it.
It went something like this:
1) Opened up Preview. Oh.. it's not editable
2) Opened up Photoshop, imported it. Photoshop converted it to 12 PNGs. Okay... Had to export each PNG back to PDF and frankenstein all 12 PDFs back to one PDF. 3 hours later, it ended up being 10x the original size.