Are signed documents legally binding?

All signed documents are legally binding.

Gust Equity Management uses HelloSign for esignatures. HelloSign complies with the requirements of the U.S. Electronic Signature in Global and National Commerce Act of 2000 (ESIGN), the Uniform Electronic Transactions Act (UETA), and the European EC/1999/93 Directive regarding electronic signatures and transmission.

For full details check out  HelloSign's Legality Statement.