Some Android file managers support WebDAV. I’m using Amaze which unfortunately doesn’t, so instead I have DAVx5 and I can use it to browse files on the server.

Alternatively, you can use rclone in Termux to download the server files to the device. I use this to sync music from my server.

For your PDF use case, unfortunately you will need to first download the file before opening it.

Copyparty! https://github.com/9001/copyparty

Insanely powerful, consumes much less resource and is very easy to setup. It does one thing (cloud storage) and it does it well.

It doesn’t have file sync, but it has WebDAV. You can mount the server as a drive. There is an account and sharing system.