The first time you email that link out, you have technologically released your ability to predict who will view the document (was the e-mail sent over secure channels end-to-end? Did it go to a trusted party who won't reshare it? Did you typo the e-mail address and send it to an undesired party? Did a recipient print it out and leave a printed copy lying around in an accessible conference room? Was the printout shredded after use? Was the shredding functionally irreversible? Is the shredding being done by a third-party that might lose some loads of documents on the way to the shred facility? Did you leave the link in your local machine's pastebuffer then walk away without locking your terminal? Etc., etc., etc.).
Even when the link is functionally unguessable, allowing non-authenticated access is just "security through obscurity."