Simply click on the image you see circled below
Then click on "Upload PDF"
Then pick the PDF from your computer and click Open and it will be imported into your File Library.
You can import any PDF into your projects quite easily. You may have created a low content interior you want to use elsewhere and this one of the ways to do so.
To see how to add one specific page then watch this video.
To see how to add a whole PDF easily then watch this video.