I tried saving my image as an image in preview and this only works if your pdf is one page. If it's multiple pages, exporting as an image will only save the first page.
The solution I found was this (it's a bit tedious, but you can get through it pretty fast once you get the hang of it):
Open PDF in GIMP (free photo editor) as an image. Only open those pages that need redaction. When you open it as an image, it essentially flattens the pdf so you're no longer able to highlight and copy text.
Use tools in GIMP to black out text.
Then, file -> print. On the print screen, choose "Print Preview". This will bring up the pdf in Preview where you can export/save the file.
If you have more than once page, then you can choose the thumbnail view in Preview and drag/drop the pdfs onto one another to form a larger document that you can then print. This is what I do when I have to sign just a few pages of a large pdf document.
Hope this helps!