One of the conditions to make e-document relevant in law is agreement entered by the parties on digital e-signature use (part 1 article 160 clause 2 of Civil Code of RF). The main purpose of the agreement is to state rules of digital e-signature use, admit digital e-signature legally equal to handwritten signature.
To interchange e-documents in Diadoc participants shall only once sign application form on accession to Diadoc. That is enough to confirm their consent to send and receive document in e-form signed with digital e-signature.
If your contractors have no technical ability or desire to receive documents in e-form, then it is necessary to conduct document flow in paper form, as electronic document flow is conducted only on a voluntary basis.