Try using the
sips -r 5 --padColor FFFFFF test.pdf
Where 5 is the degrees, FFFFFF is for white (the background) and test.pdf is your pdf. To more easily use this, try only typing
sips -r 5 --padColor FFFFFF and then drag a file on the terminal.
Edit: as user3439894 correctly points out this only works for single page pdf's and overwrites the original. You can append
--out newname.pdf to the orginal command to create a new file instead. With a multipage document you could also export single pages, run the command on individual pages and then combine them again, though that will take some effort.