

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 is awesome. You don’t even need to sync you can just mount WebDAV