To raise the cost of being able to forge a digital document beyond what an attacker is willing to pay the cost of legitimate use becomes greater than the benefit.
Exactly why we have spam. Make sending forged address and spammy e-mail require prepaid costs such that it was as expensive as snail mail and spam would drop from 90% of mail to less than 0.1%. It's only because it costs nearly nothing either directly for small quantities (like phishing scams) or via botnets to send huge quantities of spam (for bulk ads for fraudulent products including erectile disfunction drugs) that so much can be generated. We have some partial solutions but the entire solution requires better tools to manage the movement and transfer of email.
As for security of e-documents and other electronic transmissions, we need good tools to seamlessly handle authentication (proof the sender is who they claim they are) and non-repudiation (proof that you signed [or authorized] the document and that you are unable to claim you didn't - or the recipient has proof you did - authorize the document)
Repudiation is extremely critical to claim fraud or to stop a transaction, like when you claim a credit card charge is not authorized or the merchant stiffed you and you want a chargeback, or when you want to stop payment on a check. Non-repudiation is critical if you verified the recipient, provided the goods and services, and want to guarantee you get paid and the recipient can't falsely file a chargeback when they legitimately did receive the merch.
For ordinary transactions using paper documents we have notary publics who provide both and the notary affixes their seal and signature, essentially countersigning the signature for original signed documents or verifying certified copies of documents. Some organizations like stock certificate issuers require more, and so there is Medallion Signature Guarantee, where the authenticating official certifies the identity of the party to the document and agrees to cover any financial loss to the recipient of the document if the person who was guaranteed is not the correct individual (or agent if they're signing as attorney in fact or signing on behalf of an entity like a corporation or LLC.)
I am a notary public, licensed in two states. (A notary's license, or commission is, with minor exceptions only valid in the state of issue.) The jobs of a notary. in general, include the certification of documents, authentication of signatures, and taking oaths and affirmations (and some states don't include all of these or add special extras; notaries in Maryland can't certify copies of documents like contracts, while in Ohio, notaries have the power to issue subpoenas.). All done on paper and the job of a notary hasn't changed from essentially the way it was done 200 to 500 years ago. There are provisions for notaries to authenticate electronic documents but they're not uniform from state-to-state (some states don't even allow it or have the standards on how it is to be done) and the infrastructure to support it (including proper software and/or hardware for document certification and authentication) aren't there yet.
Until we have well-defined electronic equivalents like notaries public and guarantees like Medallion, we're going to continue to have a problem with those very issues of authentication and/or non-repudiation (plus security when the document must be kept private like certain contracts or secret like classified material.)