I probably get a minus here, but also Microsoft has an online viewer, which works the same way: the file needs to be publicly accessible.
Here is the link: https://view.officeapps.live.com/op/view.aspx
What I cloud add is that those files need to be publicly accessible only for a short period, i.e. until the page gets opened. So you cloud trick them by uploading the file to be viewed to a public temporary storage in a randomly generated folder and give that url to the online viewer.
Of course this is not that safe, since the file will get as some point to the temp storage and then to Google or Microsoft, but the random path names offer some degree of safety.
I've created recently a small glitch app, which demonstrates what I just explained: https://honeysuckle-eye.glitch.me/
It uploads local files to a temp storage and then opens the viewer from that temp storage; the temp storage only last for one download, so it is pretty safe.