Free PDF Encryption
1. LibreOffice offers strong, free AES encryption. You merely have to export to pdf, find the security tab, and then add a password. Text the password to your client and email the document. Then prying eyes will need your clients cell phone and the file to open the pdf.
2. If you merely want to share a file a very convenient method is through Firefox Send. You can upload a large file (2.5 gb), password protect the documents, and create a link for your viewer to go to a password protected page.
Taking your client's information confidential should naturally crossover to online activies. Frankly, encryption is a no-brainer that many clients are already used to.