The phrase "to attach a document" means to add or include a file (such as a text document, picture, or PDF) with an email or message that you are sending to someone. Full definition
There is a section at the end of the new lease that allows a landlord to attach a document with additional terms not already in the lease. (moneysense.ca)
On many occasions, they need to be able to attach documents as well. (lawyerist.com)
To back up your case, attach documents such as correspondence with the card issuer and account statements in the web - based complaint form. (creditcards.com)