How to remove scroll bars

I agree with PhilMayor​. I don’t see any reason why this shouldn’t open from an embedded Storyline block. In fact, I think the Resources tab is your best option, since it allows you to point to external_files instead of a local C drive path.

If your only concern is that the Resources tab doesn’t look aesthetically ideal, remember that you can simply disable Resources in the slide properties under Player Features. Just make sure you don’t disable it in the global player properties, as that would remove access to all imported files. You can also hide the Resources tab using JavaScript at the global level, without needing to uncheck it in slide properties.

Your path in the Open URL/file trigger should look like this:
story_content/external_files/my_file.pdf (no quotes)

This will work across all published outputs.