The only limitation is file size. I would only attach something that is required, or greatly enlightens IRS about some questionable input on a specific form. Each attachment must be linked to some form.

https://proconnect.intuit.com/support/en-us/help-article/electronic-filing/attach-pdf-tax-return-e-f...

I agree George but, IRS only sees line # and amount on efiled returns. I don't understand how they require something they don't even read.