The problem with the Internet is that it typically unencrypted. This is especially true for emails. This means when you send an email and any attachment it is sent in the clear. Not only that, your email may have to traverse a number of different systems before it reaches its final goal. This means that along the way your information could be intercepted and read without your knowledge.
The Internet was never designed to be secure so adding security can at times be very complex. If you are not using a service like Office 365 that now includes message encryption what can you do? The simplest method is probably to encrypt your attachments before sending them.
There are plenty of ways of encrypting files but one of the easiest is a product called:
which when installed on your desktop allows to to quickly and easily encrypt any file simply using the right mouse.
If you send a file that has been encrypted using AES Crypt you can be pretty sure that no one but those with the password can read the file. The recipient will also need to have AES Crypt on their system to allow decryption of the file.
The best thing about AES Crypt is that it is open source. This means that anyone can examine to the code behind the program to see exactly what it does. This removes the chances of back doors and other circumventions of the cryptography. In general, any encryption that you use should be based on open source.
Using AES Crypt you can ensure that your attachments stay private and you can also use the program to encrypt file on your desktop or laptop to prevent them from being accessed by unwanted eyes.