How to bold text on invoice?

Hey there, cichon-telus-net.

Thanks for reaching out to the Community for support. I'd be happy to provide some information,

At this time, QuickBooks doesn't have the option to change the level of boldness on invoices. I can see how this would be beneficial for you, so I recommend leaving feedback for our developers. These comments are taken into consideration when they design new features and updates for QuickBooks Online. 

If you have any other questions, feel free to reach back out here.