If you are applying for a trademark without legal representation in a country of which you are a citizen, generally, no documents are required outside of the details that have to be provided in the trademark application itself, such as the legal name of the owner, address and in some instances the nationality as well. These will have to be filled in by you.
In the case of a legal entity such as a business, the company's details will have to be filled in.
When applying through a legal representative, you might be requested to sign a Power of Attorney, which will grant the representative the right to represent you during the registration and allow them to conduct the necessary processes on your behalf.
If you are applying as a foreigner for a trademark in a country of which you are neither a resident nor a citizen, having a local representative will be required, with certain intellectual property offices requiring a form of identification such as an ID or a passport in the case of a physical person or the incorporation certificate in the case of a business.