1. Build Template action is to create Data that you can pass to GenerateAndDownloadPdfFile action or GenerateAndOpenSharePdf action and you will get PDF based on Data generated through Build Template Action.
2. If you have a screen build already that you download as PDF you need to Pass the URL of that Screen as Input to any of the two action and it will generate and download the PDF of that screen.
Let me know if you have any other doubts/isssues.