If you use a hardwired fax machine – those communications are deemed to be secure. However, if you use an electronic fax service, those faxes may or may not be secure.
If the fax is sent via email to your firm email account as an attachment, it is probably not secure. It is like sending the document as an unsecured email attachment. However, there are ways in which you can have the ease of receiving the fax electronically and maintain the security of the documents.
The electronic fax service is use sends me an email saying that a fax is available on their website. I can then view the fax and download it to my computer – in a secure manner. (This is assuming that I am using a secure wi-fi or VPN).
So check with your electronic fax provider and find out how you can make the service secure. Don’t assume that it is.