Thanks Paul. I tried this solution and unfortunately it did not work. However, I found that if you replace the download text with preview e.g.
https://xxx.instructure.com/files/xxxxx/download?download_frd=1
becomes
https://xxx.instructure.com/files/xxxxx /preview
then it works in the way I want: you get the lesson opening in a new window with no option to download or convert forms. I also found that this only works if you edit the url when you paste it into the external link dialogue. Trying to do it afterwards via the HTML editor, I found there was a lot of extra code present that I did not really understand and simply changing the url at that point failed to remove the download option.
Best wishes
Richard