1) First, copy an existing button link, such as the Apply Now button on your Join Us page, and paste it in the content of your page:

2) Clicking the link in the content editor will bring up a tooltip, click the Edit button:

3) In the edit tooltip, click the Link options gear icon :
4) This brings up the Link Editor where you can edit the link URL and text which will appear on the button:

5) To link a PDF, you can upload it to your Media Library using the Add Media button at the top of the editor. Once the PDF is added to your library, you can copy the URL from the Attachment Details sidebar :