There are multiple ways to redact PDF documents on a Mac. Here are a couple that I found useful.
PDFPen and PDFPen Pro have tools that allow you to select an area and have it fill with black. PDFPen is $50 and PDFPen Pro is $95. There is no difference between the Pro and the regular version in terms of this functionality. Using PDFPen in demo mode, because I haven't paid for a license, the program inserts a watermark on your documents. Obviously, this is not suitable for use in litigation.
That's why I chose to use ScanTango. ScanTango is a pretty usable application I came across. It costs $99 or $149, depending on the type of scanner you have to work with. I found the redaction tool worked pretty well. Actually, it even seemed to muck up the text in a live pdf document so you couldn't copy what was under the redaction. Here's what they have to say about it:
There are times when you may wish to send a document to a colleage, but need to selectively protect sensitive information. The solution is redaction. ScanTango allows you to select any area of a TIFF or PDF page and either Erase (white out) or Redact (black out) areas. And for you PDF-savvy lawyers out there, ScanTango even allows you to convert those hackable PDF pages into non-hackable flat Bitmap pages before saving and sending the document, so nobody can hack beneath your redactions..
I can't say how much I'd trust this feature with sensitive live text. I scanned all of my documents to PDF with no OCR before I did my redaction. Then, I printed to PDF to add one more layer of separation to make sure nobody could see what was behind the redactions.
So far as I can tell, ScanTango doesn't impose many limitations on the demo mode of its software. There are other ways to redact PDFs on a Mac without using Satan Adobe, but these are the simplest I have found. I'm actually kind of bummed there's no freeware or open source way to do this - or to bates stamp PDFs for that matter. These tasks seem simple enough. I don't understand why the only tools to do these kinds of things cost from $50 to hundreds. Heck, I'd even settle for a $10 or $20 solution, but there's no way I'm paying hundreds for these simple tasks. Oh yeah, it's because rich lawyers are about the only ones who need this kind of stuff. Got it, nevermind.