How to fax PO to vender

If  you haven't already, confirm that they cannot receive a PDF copy via email.   Some companies don't allow this, though most do.

If you really must FAX it, you have the option of printing the PO and use an old fashion FAX machine.  Or, you can sign up for one of the Online FAX services. 

Some services allow you to  Email directly out of QBs or using your regular email.  Others require you to upload a PDF to their system.  Basic difference is the cost, so consider how often you'll be doing this and do you need the online service. 

NOTE: A number of posts refer you to the QBs Apps.  But so far I've yet to be able to find a FAX service in Apps.  A search for "fax" still does not show anything.