This is usually due to the document in question being image-heavy. Most traditional fax machines don't handle printing images well, and as a result, the fax prints out black or garbled. What we suggest in this case is send a high-resolution color fax.
You can send a fax this way by checking the small check box under the fax number field Send a high resolution & color version of this fax. This box will appear after you've uploaded the document and entered in your recipient's fax number.
What we do is send a regular fax with a cover sheet added that will contain a link the recipient can use to go online, enter the passcode, and view a high resolution or color image of the fax. From here, they can download the image of the fax as a document.
