Look at the PDF carefully before sharing it. Most academic publishers put a timestamp on it that reveals who downloaded it, at least at institution level. Sometimes this is even embedded as metadata. If the PDF says anywhere “author personal copy”, please don’t share it on the author’s behalf.
This is mostly to avoid getting them into trouble.
Otherwise, go and share, authors love it!
There are some managed tunnel alternatives but they are not cheap. At this point it’s cheaper to rent a $20/year VPS and install one of these: https://github.com/anderspitman/awesome-tunneling
I personally love frp
Edit: I have never tried it, but there’s also Tailscale Funnel https://tailscale.com/kb/1223/tailscale-funnel/