Ideally you’d fill out the template you see in the editor when you go to create a topic or post. You can find copies of the appropriate template here:
As for your issue, we’ve had some trouble with PDFJS disrespecting the settings. Kind of a cowboy like that. Fortunately we have this issue logged and will be releasing a toggle-able switch for PDFJS that will disable (or enable) it in Brave in an update.