In the unlinked in the back section of your site, you create a file, upload the pdf into the file, and create a link for the pdf. that you can then use in a button, or in any communications (blog post, email, page) on your site so that, one someone clicks on the active link, your pdf will open.
Here's a good YouTube video that shows us how it's done: https://www.youtube.com/watch?v=RgZL9cBXckk
Hope that helps!