If you need to share a document with an external third party, you should do so securely. Sometimes password protecting and emailing the document is not such a good idea (see Why you shouldn’t password protect your Office Documents). You might consider instead giving them temporary access to the folder itself (see How to share a folder in SharePoint with external users).
But, in some circumstances, it might be necessary to email the file. Password protecting the documents themselves is an option if you have Word or Excel. But what if you want to send something that can’t be easily edited like a PDF? You usually need a paid subscription for a PDF editor to do this. Passwords may also cause other problems with the files. Plus, if you protect your originals and forget the password, you may be stuck.
7-Zip is a free archiving tool that has ability to add a password to a Zip file, keeping the originals intact. Sending a password protected Zip file also allows you to easily package multiple documents under a single secure password.
You can install 7-Zip using Ninite
Once installed, right click the file you want to send and go to 7-Zip > Add To Archive
In the next box, you have the usual options to rename and choose where to save, but you also have the option to set a password.
Change the Archive format to Zip for better compatibility so your recipient doesn’t need to install 7-Zip to open the file (though they might want to!)
Revert the encryption method to AES-256 which is very strong. Set the password.
Always send the password separately and via different medium! If you’re emailing the Zip file, consider sending the password via text. Do not email it to the same address.