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Please note that our PDF engine is not compatible with emojis and some special characters, displaying a box like that when it is not able to render a specific letter (e.g. those from Arabic script that requires a different PDF engine), specific currency symbols (we have a setting to extend compatibility with this for specific countries/regions), etc.
In brief, to fix it, you should have to remove the non-compatible characters and opening the PDF invoice again to check that the box disappeared.