I wish to apply a keyword to multiple documents. Selecting those documents and dragging a keyword into the keyword pane should, I believe, do that. In my version ( 2.9.0.42 Beta (861203) 64-bit - 11 Mar 2020) this appears to apply the keyword only to the first document.
I tried to search here for info on whether this is a known, yet-to-be-implemented feature but was unsuccessful.
The inspector doesn’t operate on multiple selections, only on the current document in the active editor. Try dragging the keyword onto the items you’ve selected in the binder instead.
Ah. That works fine. Many thanks.
There’s also a (very slightly) quicker way:
With multiple documents selected in the binder, right click on the keyword(s) you want to apply and select ‘Apply Keywords to Selected Documents’. You can use this context menu to remove keywords from multiple documents as well.
BTW, if you click on a keyword then on the magnifying glass you can search all documents for that keyword (or for the underlying word itself, or for documents with no keyword) and it will collate all the matches in a temporary collection in the binder.
Many thanks - MUCH later than intended. I’ve been writing rather than reading the forum. 
THANK YOU. I’m trying to figure out themes for a project and being able to add multiple keywords and see all files with a specific keyword have evaded me until now.