You can add additional messages in the document PDF footer/header.

To add customized message to your quotation, sales order, invoice or shipment, go to the document, click "Create PDF"
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Click Expand on Header or Footer Message.

Click on Add New to create new Header/Footer message template. you can save them for later use.
Choose the Header/Footer message for your PDF.

Once done adding the additional message(s), click Preview PDF to see if the message is placed in the right spot, click Save Customization and it's done.