As the in-app help file is unfortunately not searchable by default here is the trick -
the OmniOutliner in-app help file is an html file so you can drag&drop the OmniOutliner help icon top left in the opened help file onto the Safari icon and woop - now the in-app help file is opened in Safari and is searchable with the default page search function of Safari.
And now you can export it from Safari as an also searchable PDF and on top you can add your own annotations and all that stuff that is possible with a PDF.
You’re right if you drop it onto the safari icon in a finder window.
But if you drop the Omnioutliner help icon onto the safari icon in the dock - hope you have one there - it will work.
Or control-click on the OO help icon in the upper left corner and copy the link - open a new safari window and paste the URL in the address field ‘enter/return’ - et voilà.
Or control-click on any other button on the darker left side of the OO help window (cursor will change to a hand) copy the link and paste it into the address field of the new safari window (or tab) and (enter/return) and it will open the related chapter of the OO help immediately. Ain’t that good?
Or drop the icon through your window or to the floor and stomp… - ah, no, no, don’t do that - that is the solution for a completely different problem (-: