Yes. Sync is opt-in and off by default: you turn it on per group of documents, and nothing is uploaded until you do. What the server can read depends on how your key is held — set an encryption passphrase and your documents are end-to-end encrypted, because only you hold the key. If you let Bracepad manage the key instead (the quicker option, and the default for a Google sign-in), documents still travel and rest as ciphertext, but the server holds the key, so that is not end-to-end encryption. You are told which mode you are in, in Settings.