The text is not added directly in the invoice template. It is part of the background PDF used for generating the document. To remove it, you need to use your own background PDF without this notice.

You can configure the background PDF via TypoScript constants or the corresponding Site Settings. Depending on the PDF type, you can set the following options:

themes.configuration.extension.shop.pdf.product.background = EXT:your_extension/Resources/Private/Pdf/your-background.pdf
themes.configuration.extension.shop.pdf.invoice.background = EXT:your_extension/Resources/Private/Pdf/your-background.pdf
themes.configuration.extension.shop.pdf.return.background = EXT:your_extension/Resources/Private/Pdf/your-background.pdf
themes.configuration.extension.shop.pdf.storno.background = EXT:your_extension/Resources/Private/Pdf/your-background.pdf
themes.configuration.extension.shop.pdf.deliveryNote.background = EXT:your_extension/Resources/Private/Pdf/your-background.pdf
themes.configuration.extension.shop.pdf.sepaMandate.background = EXT:your_extension/Resources/Private/Pdf/your-background.pdf

Make sure that you do not use the default background PDF if it contains the notice. Instead, place your own PDF file in your sitepackage extension and reference it in the configuration.

Example:

themes.configuration.extension.shop.pdf.invoice.background = EXT:sitepackage/Resources/Private/Pdf/invoice-background.pdf

After that, your custom background PDF will be used for the invoice and the text will no longer appear.