In Pages v7.3, visit View menu : Customize Toolbar… and on that panel, drag and drop the Send a Copy tool onto your Toolbar before the Format button. Once you have done that, and you click on the Send a Copy tool, Apple Mail will be on that menu, and it will give you a choice of PDF to send as an attachment.

As there is no Stationery functionality in Mojave Mail, you are done with that concept. Instead, create your stationery effect in Pages via background artwork (Arrange menu : Section Masters : Move Objects to Section Master) with reduced opacity so your text will have strong emphasis in the foreground. Save that as a User template, so all you have to do is add your text. Using the first paragraph, Mail your PDF "stationery" to others. This is a word processing document solution, but you can also use the page layout document format as well. You can also incorporate Word templates from the Internet.
As Peter pointed out, sending a Pages document to others, most of whom cannot open it is bad manners.