The best way to create a PDF in Relay.app is to first create a Google Doc, which you can do in a few different ways:
Create a new document
Write additional content to an existing document
Copy an existing document (and optionally fill in placeholders, see Creating Templated Documents for more detail)
The menu of automations for Google Docs
Once you have a Google Doc, you can use the "Export document" automation, which allows you to export the Google Doc as a one of a variety of filetypes (e.g. PDF or Microsoft Word).
The export document menu
Once you've exported the document as a pdf, you will be able to use it as an attachment in emails, upload it to file storage, etc.