A PageBuilder page that is marked as reusable can be embedded into an email. To create a reusable page:
  1. Content > PageBuilder 
  2. Create a new page
  3. On step 1.6, Page Properties, check the box next to " This is a reusable Page that can be embedded in an HTML Component in other Pages."
  4. Continue as normal developing the page
Now you have a reusable page that can be embedded onto another PageBuilder or CMS page as a component. Here is how you embed the page into an email:
  1. Create a new email message (through a campaign or quick email)
  2. Go to step 4, HTML Content
  3. When using the visual editor, click on the gear symbol called "components"
  4. For component type, choose "content"
  5. For component, choose "Reusable Page"
  6. Choose the page you created and click "insert"
The actual page content will not be queried until the HTML renders when you send the message, so that component will always reflect the latest changes to the reusable page.