Please go to WordPress Dashboard > Media and upload your PDF files there. Then select the PDF file from the media section and copy the URL of the file:
Then, go to the page in question and use the Cornerstone page builder to add a Button element. In the element options you will see a section that you can add a link to the button:
Paste the URL of the PDF file into the URL option. And alternatively, you can click on the New Tab option to force the PDF file to open up on a new tab.
Kindly open up new threads for additional questions as it will help us to focus on each issue and give you a better support which you deserve. Having a long thread makes the maintaining job harder and also it will be harder for the other customers to find the correct information if they have similar issues. You are always welcomed to reply to this thread to follow up the same question.