In the UDL window (Language > Define Your Language) there’s a field near the top marked "Ext. ". Type in the file extension you want to be associated with your UDL. You can use a common one like txt, or make up your own - but don’t include the dot.

In your case, just enter adoc.

As for how to make N++ start in a UDL - I don’t think there is a way to do that.

At some point the format was changed, maybe that is the problem.

http://docs.notepad-plus-plus.org/index.php/Auto_Completion#How_to_create_keyword_auto-completion_definition_files

I think what you’re looking for is auto-completion.

See Settings -> Preferences -> Auto-Completion.

http://docs.notepad-plus-plus.org/index.php/Auto_Completion

I think you can probably stay away from most of the options for now. Just

Open the HTML file(s) in N++.
Edit.
Save.
Use a web browser to view how the web site looks.

https://www.youtube.com/watch?v=Aym6TgwowWY

Mine highlights.
http://i639.photobucket.com/albums/uu118/DeathSpecteRsBucket/th_Untitled2_1.jpg

Understood, thanks dail.