Apparently, you cannot use Windows Fax and Scan to send a fax from your PC without connecting a phone line (and possibly a modem) to your PC.
If you wish to send a fax from your PC - whether a Word document or a PDF file - there are numerous third-party companies that provide this service, where you can upload a file and then the third-party service will send it as a fax.
I cannot speak to the document security of any of these companies.
I can say that nearly all of them advertise that you can send "free" faxes, but then ask you to pay a monthly or annual fee, often $49 - $129! So the "free" advertisement is patently false. Obviously installing software from a third party to your computer carries the usual risks.