First Save the PDF to MyDrive. Then go into MyDrive find the PDF and click on Share. Then click on Advanced. Where it says "Who Has Access Click Change" change it to ON-Public on the Web and click Save. 

Then where it says Link to Share: Copy that link. Then click Done. 

Then go into Software Settings>Landing Page Content>the section where you are going to add the PDF as a link (example Section A) Highlight the area you want them to be able to click for the link then in the toolbar click on the link icon paste the link in the URL click done and it should be linked to the landing page.