How do I get a signer to send supporting documentation, like their ID or proof of income?

You can add File upload fields to a document when setting it up. If these are flagged as Required (mandatory), the signer will have to upload a file in order to Submit. 

When adding the field, make sure to enter a helpful description in the File to upload field, so that the signer knows exactly what is needed.

You can add any number of File fields to a document.