To upload any file (up to 20MB):
Open a page and EDIT the content area.
Insert a BUTTON block (look under FILTERS & LISTS) on any page of your site.
In CLICKTHROUGH URL > CLICK TO ADD URL...
Choose FILES > NEW FILE > and ADD A FILE. This gets the PDF into the website's storage and creates the link you need.
Now the filename and path will show in the CLICKTHROUGH URL field. Select this text and you have your link for anywhere on the website.
It will look something like this: /s/LIFE-Run-Poster-2016.pdf
Note: if you want to use this link anywhere outside the website, you must add the full website address (http://www.beachschoolfoundation.org/...). Wait Until this Squarespace installation is connected to your domain before trying that.