To send faxes directly from Google Docs, you'll first want to get the HelloFax for Google Docs add-on here.
Then, open the Google document you want to fax. Once you've activated the add-on, you can use it by going to Add-ons > HelloFax.
When you open the HelloFax for Google Docs add-on, you’ll see the HelloFax sidebar appear on the right. Enter the recipient's fax number, enter text to include an optional cover page, then click Send. You’ll receive a confirmation email indicating that the fax was successfully sent.