If they don't have a fax, they still have a mailing address. If you can print out the document, you can alter it before sending it back. But going outside the normal workflow invites human scrutiny, so anything you do there has to be above-board.
You could always add some words to your digital signature, like "this is not a contract", or "I refuse these terms", if that's the only way you could otherwise edit the document. If it's an automated system, it's likely that there is no human review until the documents are actually needed for some legal reason. By that time, you'll already have your money.
You could also try hacking the document system. The worst that could happen is that you don't get to work there. Second worst is that months or years into your new job, some HR person notices a discrepancy, and wants you to sign a fresh copy, or you're fired immediately. Try sending back a document that has lorem ipsum generated text or Markov chain text with the appropriate paragraph lengths? They're probably counting on you just clicking the buttons, maybe typing out your full legal name, and automatically filling the checkboxes with green checkmarks. Putting all of it into an automated system is already a means of cutting corners on legal paperwork. You can easily outsmart the computer, if there's no human helping it.