How do you add an attachment to a return?

[CesarJ]

In TurboTax Online, you can add attachments such as PDFs or images when prompted during your return to support certain entries.

To add an attachment:
1. Select the option to upload a supporting document when prompted.
2. Choose the file from your computer and upload it.
3. Confirm the upload.

Important: Attachments required for filing are generally not submitted electronically with your e-filed return. You will need to mail those documents separately to the IRS or state tax office. Keep copies of your attachments and mail them as instructed after submitting your return.

[Mike9241]

with desktop versions, you can't attach documents prepared outside the app. some can be mailed using Form 8453

https://www.irs.gov/pub/irs-pdf/f8453.pdf