There are apps available on Google Play. I personally use an app called FaxFile, but there are other options available. Keep in mind that there is a charge to send a fax from your phone using these apps.
Also, note that you can only send certain file formats. With respect to the FaxFile app I use, that means PDF, .doc/.docx, PNG, and JPEG. To send a fax of an existing document, you will either need to scan it and convert to PDF (there are free Android apps for this as well) or in a pinch you can take a photo with your phone and just fax the photo. I'd go with the PDF option to improve legibility for the recipient, personally. It may differ with other faxing apps, but I'm not familiar enough with the other apps to be sure.