If you want good cheap (free) communication privacy, then use the following methodology:
1. Use Zangi.com as an anonymous communication platform. More app options here.
2. For extra protection just attach a password protected Zip file from 7-zip.org with your document.
3. Relay a password securely by using a password manager like Bitwarden.com's Send feature.
Zangi is an encrypted communication app that provides text messaging, voice and video communication. You can attach files to a text message. No personal information is required at Zangi.com. It uses end-to-end encryption. You can send expiring messages. Add a password protected Zip file if worried. If the authorities get your phone, then they can only see encrypted Zip files instead of actual messages.
Alternatively, you could just attach password protected Zip files to your emails. This provides end-to-end encryption. But with emails comes the Metadata problem: The sender's email address, subject, time and date and more are saved by the email provider. Email Metadata is going to kill you if the authorities get a hold of it. For top level protection, the entire email universe is out. ProtonMail (Proton.me) is not going to save you.
Note that ProtonMail is never a great option. That's because most likely you will be sending emails to unencrypted addresses. Your entire email will be exposed. ProtonMail really only works if the recipient also has a ProtonMail address.
The only true end-to-end encryption across the email universe is by using attached password protected Zip files.
Now we are left with the problem of sending passwords securely. The free version of Bitwarden.com's password manager has a Send feature for sending sensitive (password) information. It gives you a one-time use link to the sensitive information. Send the link via an expiring message on Zangi.
ProtonPass also includes a way to send passwords securely, but that is included in the premium plan.
"So every few months you see prosecutors quoting a criminal’s Signal messages in some indictment or arrest warrant—leading to a round of frenzied speculation online that law enforcement has found a way to “crack” the encryption on Signal and break into people’s messages. This is not usually what’s happening. In most cases, law enforcement is arresting them, seizing their phones, opening them with a warrant, and reading all of their super-secure “encrypted” messages right there on their phone screen because they gave no thought to deletion." Source: Means of Control: How the Hidden Alliance of Tech and Government Is Creating a New American Surveillance State: Tau, Byron
So what good does all that end-to-end encryption do when the cops get your phone and read all your messages? You always have to assume that the cops will get your phone and everything is fair game. But they can't read it if it gets automatically deleted. Or everything is stored in password protected Zip files. Just use an easily remembered password or passphrase. Here is an example of a passphrase: We-go-trolling-for-starfishes-off-coast8.
The cops can force you to open your phone via fingerprint. They can't force you to cough up passwords.
Let's say your emails are encrypted and the cops can't read it. How much will that help you? Well, you're still in trouble.
"The U.S. government "kill[s] people based on metadata," but it doesn't do that with the trove of information collected on American communications, according to former head of the National Security Agency Gen. Michael Hayden."
"Hayden made the remark after saying he agreed with the idea that metadata - the information collected by the NSA about phone calls and other communications that does not include content - can tell the government "everything" about anyone it's targeting for surveillance, often making the actual content of the communication unnecessary." Source: Ex-NSA Chief: 'We Kill People Based on Metadata' - ABC News