jq is meant to be used from the command line - you should be able to invoke it on a local file just fine; in fact that’s the way it’s supposed to be used.
For instance, if I have jq for Windows downloaded, I can run this in a command prompt or other terminal window:
jq-win64.exe 'walk( if type == "object" and .valueSource == "opc" then .tooltip = "<html>TagName: \(.name) <br>Source: \(.opcItemPath)" else . end)' path/to/tag.json > tags_with_tooltips.json
The > tags_with_tooltips.json is ‘capturing’ the output buffer of the command and writing it to a new file.