Based on my quick tests, it doesn’t look like it’s currently possible to insert a PDF directly into a Writer document as an image. I was testing on 3.4.4, so it’s possible that the Feature Fairy has made your wish come true in 3.5 or 3.6.
I’m not sure how difficult it would be to add this feature, but it wouldn’t hurt for you to ask. Feature Requests can be filed as a bug report here: https://www.libreoffice.org/get-help/bug
WORKAROUND: For the short-term, I’d just use the ‘convert’ utility (a part of the ImageMagick package) to turn your PDFs into image formats that LO can consume. It looks like you’re on a Mac, and I think that most OSX and GNU/Linux-y platforms have convert installed by default. You’ll want to do something like this:
$ convert my-sweet.pdf sweet-pdf-as-image.png
If you’re working with something with fine detail (like scanned pages), you might want to bump up the quality and tweak some other parameters. Like so:
$ convert -interlace none -density 300 -quality 80 my-sweet.pdf sweet-pdf-as-image.png