With the new image/file editor, to link to a pdf to open in a new window:

highlight text that will be the link and click on the link icon.

when the dialog box opens, first go to Web URL and turn on the toggle for “open in a new window”

Then go to the Files and either upload a new file, or select an existing file (select and click on the file)

Recheck that the Website URL is still toggeled to open in a new window

Save, save.