The solution? Encrypt your documents.
Encryption is as old as people having secrets from each other. Computers and technology have made encryption more powerful and easy to use at the same time. Using encryption software, you can encrypt a file on your computer, and send it to your friend or colleague, who can decrypt the file using a password.
A simple and free solution to encrypt your documents and email attachments is called AxCrypt. You can download AxCrypt from the manufacturer's website. Make sure you download the right version for you Windows system architecture 32-bit or 64-bit.
Once you have installed AxCrypt on your system, you can right-click any file in the Windows Explorer, and select AxCrypt in the popup menu.

There are a number of options in the AxCrypt menu. The Encrypt option will encrypt the original file, and add the extension .axx. The Encrypt a copy menu option will create a copy of the original file, encrypt it and add the extension .axx. Encrypt copy to .EXE finally will create a self-decrypting version of the file, which will make it easier for the recipient of the file to decrypt it.
The simplest method of encryption will ask the user for a passphrase to use for encryption.

You can let the program remember the passphrase for decryption (only useful on your local computer of course), and you can let the program use the entered phase phrase as the default for other the encryption of other files.
Once encrypted, the files will have a file extension .axx and right-clicking a file with the .axx extension will show a Decrypt option in the AxCrypt popup menu. You will need to enter the passphrase when you decrypt an encrypted file, so make sure you share the passphrase with the recipient of the file. Best way is to tell them verbally or by phone, use a mobile text message, as that will prevent interception by anyone. The program will ask the user where the decrypted file should be saved.
AxCrypt uses 128-bit AES encryption, so it offers a decent level of protection. If you want to improve the level of protection, you can use a Key-File instead of a passphrase for encryption. This is a small file with random data in it, which ensures better security than a passphrase. AxCrypt will automatically generate a key-file for you.
So the next time you send an attachment with personal or sensitive data in it, don't rely on your ISP for a secure connection, but instead consider encrypting the document with AxCrypt or a similar solution.