To embed a PDF onto your template you'll first want to access the template by going to Templates > My Templates > Click on the template > click customize to bring you to the template editor.
While in the editor navigate to the page/group that you wish to add the PDF. You'll want to make sure you have a brand new element to add the PDF to. If you do not have one this help article will show you how you can add a new element.
Click in the empty cell within your template. Click on the + to bring up the New Element Panel.
Choose PDF Viewer under Graphical Elements.
Choose your PDF you want to embed onto the template by clicking on the browse button. You'll then be able to choose any PDF from your files.
Once your file is selected click on the Create button.
Your PDF will now be embedded onto the template.