To encrypt a file with ETER you will need to prompt the sharing menu. It is recommended to use PGP Encrypt extension regardless of the initial file location - Photos, Documents by Readdle, Apple Files etc. Then ETER will prompt you for the recipient(s). If you intend to sign the file, just click on your private key twice: "[signer]" should appear next to its name. After that, click "Encrypt", authenticate, if necessary, and distribute the encrypted file by using any available app through the standard sharing menu.