I guess it depends on how you do it, and what it is you're promoting. I'm no PR expert (in the slightest), but one method I personally find effective is to write a press release that could easily be published as a blog post "as is" with different promo images attached. I will not publish this on any of my own platforms.
I will then hunt down and make a list of websites, bloggers, vloggers, etc, that are relevant to what I'm trying to promote. Then send them each personalised email attaching the PDF and images. I've found it a good way of getting it out there in front of the right audience and getting backlinks from relevant websites (especially when run with some sort of incentive like a competition). Many bloggers will thankfully re-write the original press release anyway.

