Q: How to make Export to PDF keep the extension

Open finder preferences.

Go to "advanced" tab

Check Show all file name extensions

It will help you.

Unfortunately, Pages v5, and now v6 on macOS Sierra provide no means to export documents with their extensions unhidden. Although there is advice that one should change the Finder preferences to reveal ALL extensions on every filesystem object, I personally disagree due to the added clutter.

I prefer to keep a small AppleScript on my Desktop that allows me to select one or more documents with hidden extensions, and then just click the script, and it instantly unhides the name extension on every selected document.

One copies and pasted the following AppleScript into their open Script Editor (Launchpad : Other : Script Editor), and clicks the hammer icon to compile it. Then it is saved twice: 1) as File format: text which puts an AppleScript extension on the current document, and 2) Option key : File menu : Save As... File format: Script, or Application to the Desktop, or in your Dock.

Code:

-- se.applescript

-- Select the file(s) in the Finder whose extensions are hidden. Run this script to

-- show the extensions.

-- VikingOSX, Sept 24, 2016, Apple Support Community

tell application "Finder"

       set sel to selection as alias list

       if sel = {} then

            display dialog "Select the files in the Finder whose extensions you want displayed."

            return

       end if

       repeat with afile in sel

            set extension hidden of afile to false

       end repeat

end tell