In that case, you can use https://www.pdftool.org/, which runs in the browser but offline and never uploads your files to any server.
THe site renders fine otherwise. I'm not a technical user, but i do run Ublock in the complete Javascript disabled settings.
Should security concerns really warrant not trusting the (reputable) vendor that the files are not being uploaded, you would need to do some sort of audit and/or run in an isolated environment and wouldn't be the "random user" referred to in OP.
If you also wear a tinfoil hat, delete the local storage, etc, after you are done using it.