When you print your return to mail it to Canada Revenue Agency (CRA), not all your forms are printed. Instead, they're "condensed" in the barcodes. This allows CRA to process your return faster and with greater accuracy. The PDF that says “duplicate do not send by mail” is for your reference only.
If you are a non-resident, you can also file your tax return via FAX. You can find the correct number to use on this Canada Revenue Agency (CRA) page: https://www.canada.ca/en/revenue-agency/corporate/contact-information/where-mail-your-paper-t1-return.html#nonres